checkcopies: handle divergences contained entirely in tca::ctx

During a graftlike merge, _checkcopies runs from ctx to tca, possibly
passing over the merge base. If there is a rename both before and after
the base, then we're actually dealing with divergent renames.
If there is no rename on the other side of tca, then the divergence is
contained entirely in the range of one _checkcopies invocation, and
should be detected "in the loop" without having to rely on the other
_checkcopies pass.

mercurial/copies.py

@@ -564,9 +564,8 b' def _checkcopies(ctx, f, m1, m2, base, t'
     # traversed backwards.
     #
     # In the case there is both backward and forward renames (before and after
-    # the base) this is more complicated as we must detect a divergence. This
-    # is currently broken and hopefully some later code update will make that
-    # work (we use 'backwards = False' in that case)
+    # the base) this is more complicated as we must detect a divergence.
+    # We use 'backwards = False' in that case.
     backwards = base != tca and f in mb
     getfctx = _makegetfctx(ctx)
 
@@ -600,6 +599,12 b' def _checkcopies(ctx, f, m1, m2, base, t'
                 data['copy'][of] = f
             elif of in mb:
                 data['copy'][f] = of
+            else: # divergence w.r.t. graft CA on one side of topological CA
+                for sf in seen:
+                    if sf in mb:
+                        assert sf not in data['diverge']
+                        data['diverge'][sf] = [f, of]
+                        break
             return
 
     if of in mb:

tests/test-graft.t

@@ -982,6 +982,9 b' Test the cases A.0 (f4x) and A.6 (f3x)'
 
   $ HGEDITOR="echo D1 >" hg graft -r 'desc("D0")' --edit
   grafting 3:b69f5839d2d9 "D0"
+  note: possible conflict - f3b was renamed multiple times to:
+   f3d
+   f3a
   warning: can't find ancestor for 'f3d' copied from 'f3b'!
 
 Set up the repository for some further tests