aggressivemergedeltas: enabled the option by default

The option has been around for a while (August 2015) but was never turned on by
default. In-depth testing shows large wins for having that on with no
significant drawbacks.

When enabled, revlog consider delta against both p1 and p2 at the same time when
storing a revision. Selecting a delta against "p2" can produce better deltas and
chain. This raise large benefit for all repositories, especially if they have a
lot of merges.

Comparison of `.hg/store/` size:

    mercurial (6.74% merges):
        before:    54,225,348 bytes
        after:     47,279,959 bytes -13%
    pypy (8.30% merges):
        before:   459,041,759 bytes
        after:    346,090,067 bytes -25%
    netbeans (34.21% merges):
        before: 2,468,041,333 bytes
        after:  1,364,077,645 bytes -45%
    mozilla-central (4.84% merges):
        before: 2,731,799,546 bytes
        after:  2,157,718,019 bytes -21%

Comparison of `00manifest.d` size:

    mercurial (6.74% merges):
        before:    11,682,516 bytes
        after:      6,143,044 bytes -47%
    pypy (8.30% merges):
        before:   156,447,163 bytes
        after:     52,941,780 bytes -66%
    netbeans (34.21% merges):
        before: 1,250,363,851 bytes
        after:    130,088,982 bytes -90%
    mozilla-central (4.84% merges):
        before:   468,202,733 bytes
        after:    215,096,339 bytes -54%

In addition, the better deltas help with the performance of multiple core
operations. However, better chains mean longer chains, which can affect
performance negatively (mostly manifest revision retrieval time). Chains length
is a deeper problem that also affects linear repository too. Overall we think
the benefits of using p2 as a diff target are bigger than the downsizes. In
addition, we are also working on ways to improve the performance impact of chain
length, so theses downsizes get fixed in the future.

Below are interesting items from the full benchmark run:

    bundling 100 revisions from pypy:
        before: 670ms
        after:  480ms -28%
    bundle 10000 revisions from pypy:
        before: 1.38s
        after:  1.10s -54%
    bundle 10000 revisions from pypy:
        before: 16.1s
        after:  7.81s -52%
    bundle 10000 revisions from netbeans:
        before: 19.3s
        after:  15.5s -19%

    unbundle 1000 revisions to pypy:
        before: 641ms
        after:  315ms - 51%

    clone mercurial (http):
        before: 26.0s
        after:  22.6s -23%

    pulling 1000 revisions from pypy (shh):
        before: 2.07s
        after:  1.36s -44%

    pushing 1000 revision through http (pypy repository)
        before: 2.18s
        after:  1.35s -48%

    diff time in mozilla-central:
        before: 1.420s
        after:  0.983s -31%

    status time in mozilla-central:
        before: 1.260s
        after:  0.828s -34%

Impact in other cases seems minimal (within a couple of percent in worse cases)
and can be seen in both direction:

Timing for a simple `hg commit`:

    mozilla-central:
        before: 3.37s
        after:  3.22s -4%
    pypy:
        before:  194ms
        after:   197ms +2%

Timing for status (from tip to parent of tip):
    mercurial:
        before: 52.4ms
        after:  52.4ms (same)
    pypy:
        before: 55.2
        after:  56.9 +3%

Timing for `hg update`

    mozilla-central, across 10 revisions:
        before: 4.82s
        after:  4.59s -5%
    mozilla-central, across 10000 revisions:
        before: 49.1s
        after:  49.9s +2%
    pypy, across 10 revisions:
        before: 213ms
        after:  216ms +1%
    pypy, across 10000 revisions:
        before: 5.31ms
        after:  5.24ms -1%

The negative consequences are related to manifest fetch time:
(timing for the tip revision tested by the benchmark)

    pypy-2018:
        before: 2.60ms
        after:  3.88ms +50%
    mozilla-central-2018:
        before: 565ms
        after:  652ms +15% (~+100ms)
    netbeans-2018:
        before: 101ms
        after:  250ms +48% (~+150ms)

This shows up as a fixed overhead on some command we benchmarked:

    no-op push of mozilla-central:
        before:  945ms
        after:  1040ms +10% (~+100ms)
    pushing 10 changeset in netbeabs over ssh:
        before:  557ms
        after:   712ms +28% (+155ms)
    pushing 100 changeset in netbeabs over ssh:
        before: 592ms
        after:  771ms +30% (+179ms)

mercurial/configitems.py

@@ -629,7 +629,7 b" coreconfigitem('extdata', '.*',"
     generic=True,
 )
 coreconfigitem('format', 'aggressivemergedeltas',
-    default=False,
+    default=True,
 )
 coreconfigitem('format', 'chunkcachesize',
     default=None,

mercurial/revlog.py

@@ -615,7 +615,7 b' class revlog(object):'
         # How much data to read and cache into the raw revlog data cache.
         self._chunkcachesize = 65536
         self._maxchainlen = None
-        self._aggressivemergedeltas = False
+        self._aggressivemergedeltas = True
         self.index = []
         # Mapping of partial identifiers to full nodes.
         self._pcache = {}

tests/test-generaldelta.t

@@ -139,7 +139,7 b' Test format.aggressivemergedeltas'
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
         0       1        1       -1    base         59        215         59   0.27442        59         0    0.00000
         1       1        2        0    prev         61         86        120   1.39535       120         0    0.00000
-        2       1        3        1      p1         65        301        185   0.61462       185         0    0.00000
+        2       1        2        0      p2         62        301        121   0.40199       182        61    0.50413
 
   $ hg strip -q -r . --config extensions.strip=