dirstate: use visitchildrenset in traverse

This speeds up `hg status` a fair amount when there is a very large directory
and narrow is in use.

Timing numbers according to command:

    hyperfine --warmup 1 'hg status'

HGRCPATH points to a file with the following contents:

    [extensions]
    narrow =

mozilla-unified (called m-u below) was at revision  #468856.
        regular hash: eb39298e432d
  treemanifests hash: 0553b7f29eaf

large-dir-repo (called l-d-r below) was generated with the following script:

    #!/bin/bash
    hg init large-dir-repo
    mkdir -p large-dir-repo/third_party/rust/log
    touch large-dir-repo/third_party/rust/log/foo.txt
    for i in $(seq 1 30000); do
        d=$(mktemp -d large-dir-repo/third_party/XXXXXXXXX)
        touch $d/file.txt
    done
    hg -R large-dir-repo ci -Am 'rev0' --user test --date '0 0'

for repos that use narrow, the narrowspec was this:

    [includes]
    rootfilesin:third_party/rust/log
    [excludes]

This narrowspec was chosen due to the size of the third_party/rust directory;
this directory was *not* modified in revision  #468856 in mozilla-unified.

Importantly, when using narrow, these repos had everything checked out (in the
case of large-dir-repo, that means all 30,001 directories), *before* adding the
narrowspec. This is to simulate the behavior when using a virtual filesystem
that shows everything for the user even if they haven't added it to the
narrowspec yet. This is not a supported configuration, and `hg update` will not
really do the "correct" thing, but non-mutating commands should behave
correctly.

There are two repos below that do not follow the setup above, 'citc1' and
'citc2', which are using a virtual filesystem and can not be reproduced
upstream; these numbers are here mostly to indicate that these performance
improvements are not hypothetical, and show the benefits we're hoping to achieve
on our real workloads. 'citc1' is closest to large-dir-repo with one of our
pathological cases, 'citc2' is an arbitrary repo and closer to "average".

I'm not claiming anything less than a 5% speed win as improvements due to this
change; these are probably eiter measurement artifacts or constant time
improvements. The numbers that aren't changing are shown primarily to prove that
this doesn't make anything worse in any case I plan on testing during this
series.

'before' is hg from commit c83ad576. 'N' indicates narrow in use, 'T' indicates
treemanifest in use.

    hg status:
    repo  | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before
    ------+---+---+------------------------+-----------------------+------------
    m-u   |   |   | 2.284 s +-  0.022 s    | 2.274 s +-  0.021 s   |  99.6%
    m-u   |   | x | 2.289 s +-  0.008 s    | 2.284 s +-  0.028 s   |  99.8%
    m-u   | x |   | 430.8 ms +-   3.1 ms   | 424.5 ms +-   3.2 ms  |  98.5%
    m-u   | x | x | 429.8 ms +-   2.5 ms   | 425.8 ms +-   3.7 ms  |  99.1%
    l-d-r |   |   | 681.3 ms +-   5.5 ms   | 689.6 ms +-   8.0 ms  | 101.2%
    l-d-r |   | x | 666.8 ms +-  21.8 ms   | 672.5 ms +-  14.9 ms  | 100.9%
    l-d-r | x |   | 282.6 ms +-   1.8 ms   | 203.0 ms +-   1.2 ms  |  71.8% <--
    l-d-r | x | x | 275.2 ms +-   3.9 ms   | 199.3 ms +-   3.5 ms  |  72.4% <--
    citc1 | x | x | 1.023 s +-  0.011 s    | 398.6 ms +-   9.2 ms  |  39.0% <--
    citc2 | x | x | 297.9 ms +-   4.4 ms   | 289.6 ms +-   4.2 ms  |  97.2%

    hg status --change .:
    repo  | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before
    ------+---+---+------------------------+-----------------------+------------
    m-u   |   |   | 478.2 ms +-   2.0 ms   | 476.9 ms +-   3.7 ms  |  99.7%
    m-u   |   | x | 169.5 ms +-   2.7 ms   | 169.5 ms +-   2.5 ms  | 100.0%
    m-u   | x |   | 477.0 ms +-   2.4 ms   | 476.1 ms +-   1.4 ms  |  99.8%
    m-u   | x | x | 124.7 ms +-   1.9 ms   | 124.2 ms +-   3.3 ms  |  99.6%
    l-d-r |   |   | 97.4 ms +-   1.2 ms    | 96.5 ms +-   1.2 ms   |  99.1%
    l-d-r |   | x | 4.778 s +-  0.018 s    | 4.774 s +-  0.011 s   |  99.9%
    l-d-r | x |   | 99.9 ms +-   1.1 ms    | 98.8 ms +-   1.3 ms   |  98.9%
    l-d-r | x | x | 848.7 ms +-   7.1 ms   | 849.4 ms +-   6.5 ms  | 100.1%
    citc1 | x | x | 4.250 s +-  0.051 s    | 4.283 s +-  0.042 s   | 100.8%
    citc2 | x | x | 341.5 ms +-   4.7 ms   | 341.5 ms +-   4.1 ms  | 100.0%

    hg update $rev^; hg update $rev:
    repo  | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before
    ------+---+---+------------------------+-----------------------+------------
    m-u   |   |   | 4.357 s +-  0.032 s    | 4.312 s +-  0.093 s   |  99.0%
    m-u   |   | x | 3.599 s +-  0.061 s    | 3.592 s +-  0.071 s   |  99.8%
    m-u   | x |   | 1.815 s +-  0.012 s    | 1.816 s +-  0.013 s   | 100.1%
    m-u   | x | x | 1.110 s +-  0.009 s    | 1.106 s +-  0.005 s   |  99.6%
    l-d-r |   |   | 527.1 ms +-   7.8 ms   | 523.3 ms +-   6.5 ms  |  99.3%
    l-d-r |   | x | 8.835 s +-  0.067 s    | 8.825 s +-  0.064 s   |  99.9%
    l-d-r | x |   | 313.0 ms +-   2.2 ms   | 312.1 ms +-   1.2 ms  |  99.7%
    l-d-r | x | x | 1.780 s +-  0.011 s    | 1.799 s +-  0.013 s   | 101.1%
    citc1 | x | x | 6.825 s +-  0.262 s    | 6.707 s +-  0.353 s   |  98.3%
    citc2 | x | x | 776.4 ms +-   4.5 ms   | 781.3 ms +-   6.3 ms  | 100.6%

    hg diff:
    repo  | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before
    ------+---+---+------------------------+-----------------------+------------
    m-u   |   |   | 1.519 s +-  0.015 s    | 1.525 s +-  0.017 s   | 100.4%
    m-u   |   | x | 1.512 s +-  0.010 s    | 1.517 s +-  0.027 s   | 100.3%
    m-u   | x |   | 420.0 ms +-   3.2 ms   | 417.1 ms +-   1.9 ms  |  99.3%
    m-u   | x | x | 415.0 ms +-   3.8 ms   | 415.7 ms +-   2.7 ms  | 100.2%
    l-d-r |   |   | 220.8 ms +-   4.0 ms   | 220.8 ms +-   3.7 ms  | 100.0%
    l-d-r |   | x | 216.6 ms +-   7.5 ms   | 211.4 ms +-   2.1 ms  |  97.6%
    l-d-r | x |   | 111.9 ms +-   1.8 ms   | 112.0 ms +-   1.5 ms  | 100.1%
    l-d-r | x | x | 111.4 ms +-   1.4 ms   | 110.2 ms +-   1.0 ms  |  98.9%
    citc1 | x | x | 268.7 ms +-   2.3 ms   | 269.6 ms +-   2.8 ms  | 100.3%
    citc2 | x | x | 273.5 ms +-   5.5 ms   | 273.9 ms +-   3.7 ms  | 100.1%

    hg diff -c .:
    repo  | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before
    ------+---+---+--------------------------+-----------------------+----------
    m-u   |   |   | 497.1 ms +-   1.4 ms   | 500.1 ms +-   2.4 ms  | 100.6%
    m-u   |   | x | 195.3 ms +-  13.2 ms   | 191.6 ms +-   3.0 ms  |  98.1%
    m-u   | x |   | 476.8 ms +-   1.9 ms   | 476.7 ms +-   2.3 ms  | 100.0%
    m-u   | x | x | 122.8 ms +-   2.1 ms   | 122.9 ms +-   2.0 ms  | 100.1%
    l-d-r |   |   | 99.3 ms +-   2.3 ms    | 98.8 ms +-   1.7 ms   |  99.5%
    l-d-r |   | x | 4.875 s +-  0.041 s    | 4.847 s +-  0.038 s   |  99.4%
    l-d-r | x |   | 98.5 ms +-   1.2 ms    | 98.9 ms +-   1.3 ms   | 100.4%
    l-d-r | x | x | 864.6 ms +-   7.4 ms   | 855.4 ms +-   6.6 ms  |  98.9%
    citc1 | x | x | 4.505 s +-  0.060 s    | 4.466 s +-  0.036 s   |  99.1%
    citc2 | x | x | 368.0 ms +-   4.0 ms   | 365.5 ms +-   6.3 ms  |  99.3%

Differential Revision: https://phab.mercurial-scm.org/D4131

mercurial/dirstate.py

@@ -893,8 +893,11 b' class dirstate(object):'
             wadd = work.append
             while work:
                 nd = work.pop()
-                if not match.visitdir(nd):
+                visitentries = match.visitchildrenset(nd)
+                if not visitentries:
                     continue
+                if visitentries == 'this' or visitentries == 'all':
+                    visitentries = None
                 skip = None
                 if nd == '.':
                     nd = ''
@@ -909,6 +912,8 b' class dirstate(object):'
                         continue
                     raise
                 for f, kind, st in entries:
+                    if visitentries and f not in visitentries:
+                        continue
                     if normalizefile:
                         # even though f might be a directory, we're only
                         # interested in comparing it to files currently in the