copies: move short-circuiting of dirstate copies out of _forwardcopies()

I'd like to move the filtering of copies we do after chaining to the
end of all chaining (in a single place in pathcopies()). One problem
that came up when trying that was that we allow things like `hg cp -f
<file> <existing file>` so the user can later amend that in. Filtering
at the end would mean that we remove those copies. That would break
`hg st -C`. This patch therefore moves the short-circuiting of
dirstate copies into pathcopies() so we can more easily handle the
dirstate-only case differently.

I initially thought this might change some behavior when the user does
`hg status --rev 'wdir()' --rev .` during an uncommitted merge, since
_backwardrenames() would reverse the copies in that case. However, I
couldn't come up with a test case where it made a difference.

Differential Revision: https://phab.mercurial-scm.org/D6600

mercurial/copies.py

@@ -324,10 +324,6 b' def _forwardcopies(a, b, match=None):'
     match = a.repo().narrowmatch(match)
     # check for working copy
     if b.rev() is None:
-        if a == b.p1():
-            # short-circuit to avoid issues with merge states
-            return _dirstatecopies(b._repo, match)
-
         cm = _committedforwardcopies(a, b.p1(), match)
         # combine copies from dirstate if necessary
         return _chainandfilter(a, b, cm, _dirstatecopies(b._repo, match))
@@ -367,6 +363,9 b' def pathcopies(x, y, match=None):'
     if a == x:
         if debug:
             repo.ui.debug('debug.copies: search mode: forward\n')
+        if y.rev() is None and x == y.p1():
+            # short-circuit to avoid issues with merge states
+            return _dirstatecopies(repo, match)
         return _forwardcopies(x, y, match=match)
     if a == y:
         if debug: