copies: extract method for getting non-wdir forward copies

I may add an alternative way of getting copy metadata (from changelog,
not filelog) but the chaining with the dirstate copy metadata will be
the same, so it will probably help to have this extracted. Even if
that doesn't happen, the next patch will show that we can simplify
this a bit after this refactoring, so it seems worth it regardless.

Differential Revision: https://phab.mercurial-scm.org/D1697

mercurial/copies.py

@@ -156,18 +156,8 @@ def _computeforwardmissing(a, b, match=None):
     mb = b.manifest()
     return mb.filesnotin(ma, match=match)
 
-def _forwardcopies(a, b, match=None):
-    """find {dst@b: src@a} copy mapping where a is an ancestor of b"""
-
-    # check for working copy
-    w = None
-    if b.rev() is None:
-        w = b
-        b = w.p1()
-        if a == b:
-            # short-circuit to avoid issues with merge states
-            return _dirstatecopies(w, match)
-
+def _committedforwardcopies(a, b, match):
+    """Like _forwardcopies(), but b.rev() cannot be None (working copy)"""
     # files might have to be traced back to the fctx parent of the last
     # one-side-only changeset, but not further back than that
     limit = _findlimit(a._repo, a.rev(), b.rev())
@@ -199,6 +189,21 @@ def _forwardcopies(a, b, match=None):
         ofctx = _tracefile(fctx, am, limit)
         if ofctx:
             cm[f] = ofctx.path()
+    return cm
+
+def _forwardcopies(a, b, match=None):
+    """find {dst@b: src@a} copy mapping where a is an ancestor of b"""
+
+    # check for working copy
+    w = None
+    if b.rev() is None:
+        w = b
+        b = w.p1()
+        if a == b:
+            # short-circuit to avoid issues with merge states
+            return _dirstatecopies(w, match)
+
+    cm = _committedforwardcopies(a, b, match)
 
     # combine copies from dirstate if necessary
     if w is not None: