--- a/mercurial/copies.py	Thu Apr 16 22:33:53 2015 +0900
+++ b/mercurial/copies.py	Thu Apr 16 20:57:51 2015 -0500
@@ -8,10 +8,6 @@
 import util
 import heapq
 
-def _nonoverlap(d1, d2, d3):
-    "Return list of elements in d1 not in d2 or d3"
-    return sorted([d for d in d1 if d not in d3 and d not in d2])
-
 def _dirname(f):
     s = f.rfind("/")
     if s == -1:
@@ -144,7 +140,19 @@
             del c[k]
     return c
 
-def _forwardcopies(a, b):
+def _computeforwardmissing(a, b, match=None):
+    """Computes which files are in b but not a.
+    This is its own function so extensions can easily wrap this call to see what
+    files _forwardcopies is about to process.
+    """
+    ma = a.manifest()
+    mb = b.manifest()
+    if match:
+        ma = ma.matches(match)
+        mb = mb.matches(match)
+    return mb.filesnotin(ma)
+
+def _forwardcopies(a, b, match=None):
     '''find {dst@b: src@a} copy mapping where a is an ancestor of b'''
 
     # check for working copy
@@ -167,9 +175,7 @@
     # 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 = set(b.manifest().iterkeys())
-    missing.difference_update(a.manifest().iterkeys())
-
+    missing = _computeforwardmissing(a, b, match=match)
     ancestrycontext = a._repo.changelog.ancestors([b.rev()], inclusive=True)
     for f in missing:
         fctx = b[f]
@@ -197,16 +203,36 @@
         r[v] = k
     return r
 
-def pathcopies(x, y):
+def pathcopies(x, y, match=None):
     '''find {dst@y: src@x} copy mapping for directed compare'''
     if x == y or not x or not y:
         return {}
     a = y.ancestor(x)
     if a == x:
-        return _forwardcopies(x, y)
+        return _forwardcopies(x, y, match=match)
     if a == y:
         return _backwardrenames(x, y)
-    return _chain(x, y, _backwardrenames(x, a), _forwardcopies(a, y))
+    return _chain(x, y, _backwardrenames(x, a),
+                  _forwardcopies(a, y, match=match))
+
+def _computenonoverlap(repo, c1, c2, addedinm1, addedinm2):
+    """Computes, based on addedinm1 and addedinm2, the files exclusive to c1
+    and c2. This is its own function so extensions can easily wrap this call
+    to see what files mergecopies is about to process.
+
+    Even though c1 and c2 are not used in this function, they are useful in
+    other extensions for being able to read the file nodes of the changed files.
+    """
+    u1 = sorted(addedinm1 - addedinm2)
+    u2 = sorted(addedinm2 - addedinm1)
+
+    if u1:
+        repo.ui.debug("  unmatched files in local:\n   %s\n"
+                      % "\n   ".join(u1))
+    if u2:
+        repo.ui.debug("  unmatched files in other:\n   %s\n"
+                      % "\n   ".join(u2))
+    return u1, u2
 
 def mergecopies(repo, c1, c2, ca):
     """
@@ -288,15 +314,9 @@
 
     repo.ui.debug("  searching for copies back to rev %d\n" % limit)
 
-    u1 = _nonoverlap(m1, m2, ma)
-    u2 = _nonoverlap(m2, m1, ma)
-
-    if u1:
-        repo.ui.debug("  unmatched files in local:\n   %s\n"
-                      % "\n   ".join(u1))
-    if u2:
-        repo.ui.debug("  unmatched files in other:\n   %s\n"
-                      % "\n   ".join(u2))
+    addedinm1 = m1.filesnotin(ma)
+    addedinm2 = m2.filesnotin(ma)
+    u1, u2 = _computenonoverlap(repo, c1, c2, addedinm1, addedinm2)
 
     for f in u1:
         ctx = setupctx(c1)
@@ -320,7 +340,7 @@
         else:
             diverge2.update(fl) # reverse map for below
 
-    bothnew = sorted([d for d in m1 if d in m2 and d not in ma])
+    bothnew = sorted(addedinm1 & addedinm2)
     if bothnew:
         repo.ui.debug("  unmatched files new in both:\n   %s\n"
                       % "\n   ".join(bothnew))