copies: optimize forward copy detection logic for rebases

Forward copy detection (i.e. detecting what files have been moved/copied in
commit X since ancestor Y) previously required diff'ing the manifests of both X
and Y. This was expensive since it required reading both entire manifests and
doing a set difference (they weren't already in a set because of the
lazymanifest work). This cost almost 1 second on very large repositories, and
happens N times for a rebase of N commits.

This patch optimizes it for the case of rebase. In a rebase, we are comparing a
commit against it's immediate parent, and therefore we can know what files
changed by looking at ctx.files().  This lets us drastically decrease the size
of the set comparison, and makes it O(# of changes) instead of O(size of
manifest). This makes it take 1ms instead of 1000ms.

mercurial/copies.py

@@ -10,7 +10,9 @@ from __future__ import absolute_import
 import heapq
 
 from . import (
+    node,
     pathutil,
+    scmutil,
     util,
 )
 
@@ -175,7 +177,18 @@ def _forwardcopies(a, b, match=None):
     # we currently don't try to find where old files went, too expensive
     # this means we can miss a case like 'hg rm b; hg cp a b'
     cm = {}
-    missing = _computeforwardmissing(a, b, match=match)
+
+    # Computing the forward missing is quite expensive on large manifests, since
+    # it compares the entire manifests. We can optimize it in the common use
+    # case of computing what copies are in a commit versus its parent (like
+    # during a rebase or histedit). Note, we exclude merge commits from this
+    # optimization, since the ctx.files() for a merge commit is not correct for
+    # this comparison.
+    forwardmissingmatch = match
+    if not match and b.p1() == a and b.p2().node() == node.nullid:
+        forwardmissingmatch = scmutil.matchfiles(a._repo, b.files())
+    missing = _computeforwardmissing(a, b, match=forwardmissingmatch)
+
     ancestrycontext = a._repo.changelog.ancestors([b.rev()], inclusive=True)
     for f in missing:
         fctx = b[f]