manifestmerge: fix order in which manifests are fetched

If the manifest of an earlier revision on the same delta chain is read before
that of a later revision, the revlog remembers that we parsed the earlier
revision and continues applying deltas from there onwards. If manifests are
parsed the other way round, we have to start over from the fulltext.

For a fresh clone of mozilla-central, updating from 29dd80c95b7d to its parent
aab96936a177 requires approximately 400 fewer zlib.decompress calls, which
results in a speedup from 1.10 seconds to 1.05.

mercurial/merge.py

@@ -196,6 +196,7 b' def manifestmerge(repo, wctx, p2, pa, br'
     overwrite = force and not branchmerge
     actions, copy, movewithdir = [], {}, {}
 
+    followcopies = False
     if overwrite:
         pa = wctx
     elif pa == p2: # backwards
@@ -203,6 +204,13 b' def manifestmerge(repo, wctx, p2, pa, br'
     elif not branchmerge and not wctx.dirty(missing=True):
         pass
     elif pa and repo.ui.configbool("merge", "followcopies", True):
+        followcopies = True
+
+    # manifests fetched in order are going to be faster, so prime the caches
+    [x.manifest() for x in
+     sorted(wctx.parents() + [p2, pa], key=lambda x: x.rev())]
+
+    if followcopies:
         ret = copies.mergecopies(repo, wctx, p2, pa)
         copy, movewithdir, diverge, renamedelete = ret
         for of, fl in diverge.iteritems():