merge: exit early during a no-op update (BC)

hg update . (or equivalents) are effectively no-ops in just about all
circumstances.  These sorts of updates can be especially common in a
bookmark-oriented workflow. This saves us a status check and a manifest
decompression, which means that on a repo with over 210,000 files, this brings
hg update . down from 2.5 seconds to 0.15.

There is one change in behavior: a file that was added, not committed, and then
deleted but not removed used to be removed from the dirstate. With this patch
it isn't. This is what causes the change in test-mq-qpush-exact.t. This seems
like it's enough of an edge case to not be worth handling.

The output of test-empty.t changes because those files are not yet created.

mercurial/merge.py

@@ -719,6 +719,12 b' def update(repo, node, branchmerge, forc'
                                        "subrepository '%s'") % s)
 
         elif not overwrite:
+            if p1 == p2: # no-op update
+                # call the hooks and exit early
+                repo.hook('preupdate', throw=True, parent1=xp2, parent2='')
+                repo.hook('update', parent1=xp2, parent2='', error=0)
+                return 0, 0, 0, 0
+
             if pa not in (p1, p2):  # nolinear
                 dirty = wc.dirty(missing=True)
                 if dirty or onode is None:

tests/test-empty.t

@@ -42,8 +42,6 b' Poke at a clone:'
   0 files, 0 changesets, 0 total revisions
   $ ls .hg
   00changelog.i
-  branch
-  dirstate
   hgrc
   requires
   store

tests/test-mq-qpush-exact.t

@@ -243,6 +243,7 b' qpush --exact --force with changes to a '
   +cp1
   $ hg qpop -aqf
   patch queue now empty
+  $ hg forget fp1
   $ rm fp1
   $ rm fp1.rej