localrepo.commit: grab locks before getting the list of files to commit

Somebody may change the dirstate after we've determined the parents of
the working dir and run repo.status, but before we called wlock().

This should also fix issue997, where backout would change a file without
changing its size and then call repo.commit without passing the list of
files. If this happened in less than one second, we wouldn't detect any
file changes - the in-memory dirstate still has the cached stat data for
that file. Grabbing the wlock early causes the dirstate to be
invalidated and we end up reading the dirstate file again, which has
that file marked for lookup (size == -1).

A better fix would be for backout to give repo.commit the exact list of
files, but that'll require some changes to the revert operation.

A significant user-visible change is that the precommit hook is always
run with both locks grabbed - previously, hg commit would run it before
grabbing any locks, but hg import would run it after grabbing locks.

mercurial/localrepo.py

@@ -752,6 +752,8 b' class localrepository(repo.repository):'
         if files:
             files = util.unique(files)
         try:
+            wlock = self.wlock()
+            lock = self.lock()
             commit = []
             remove = []
             changed = []
@@ -810,8 +812,6 b' class localrepository(repo.repository):'
 
             self.hook("precommit", throw=True, parent1=xp1, parent2=xp2)
 
-            wlock = self.wlock()
-            lock = self.lock()
             tr = self.transaction()
             trp = weakref.proxy(tr)