checkcopies: add logic to handle remotebase

As the two _checkcopies passes' ranges are separated by tca, not base,
only one of the two passes will actually encounter the base.
Pass "remotebase" to the other pass to let it know not to expect passing
over the base. This is required for handling a few unusual rename cases.

mercurial/copies.py

@@ -413,10 +413,10 b' def mergecopies(repo, c1, c2, base):'
                                       baselabel='topological common ancestor')
 
     for f in u1u:
-        _checkcopies(c1, f, m1, m2, base, tca, limit, data1)
+        _checkcopies(c1, f, m1, m2, base, tca, dirtyc1, limit, data1)
 
     for f in u2u:
-        _checkcopies(c2, f, m2, m1, base, tca, limit, data2)
+        _checkcopies(c2, f, m2, m1, base, tca, dirtyc2, limit, data2)
 
     copy = dict(data1['copy'].items() + data2['copy'].items())
     fullcopy = dict(data1['fullcopy'].items() + data2['fullcopy'].items())
@@ -460,8 +460,8 b' def mergecopies(repo, c1, c2, base):'
              'incompletediverge': bothincompletediverge
             }
     for f in bothnew:
-        _checkcopies(c1, f, m1, m2, base, tca, limit, both1)
-        _checkcopies(c2, f, m2, m1, base, tca, limit, both2)
+        _checkcopies(c1, f, m1, m2, base, tca, dirtyc1, limit, both1)
+        _checkcopies(c2, f, m2, m1, base, tca, dirtyc2, limit, both2)
     if dirtyc1:
         assert both2['incomplete'] == {}
         remainder = _combinecopies({}, both1['incomplete'], copy, bothdiverge,
@@ -590,7 +590,7 b' def _related(f1, f2, limit):'
     except StopIteration:
         return False
 
-def _checkcopies(ctx, f, m1, m2, base, tca, limit, data):
+def _checkcopies(ctx, f, m1, m2, base, tca, remotebase, limit, data):
     """
     check possible copies of f from m1 to m2
 
@@ -600,6 +600,7 b' def _checkcopies(ctx, f, m1, m2, base, t'
     m2 = the destination manifest
     base = the changectx used as a merge base
     tca = topological common ancestor for graft-like scenarios
+    remotebase = True if base is outside tca::ctx, False otherwise
     limit = the rev number to not search beyond
     data = dictionary of dictionary to store copy data. (see mergecopies)
 
@@ -619,7 +620,7 b' def _checkcopies(ctx, f, m1, m2, base, t'
     # 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.
     # We use 'backwards = False' in that case.
-    backwards = base != tca and f in mb
+    backwards = not remotebase and base != tca and f in mb
     getfctx = _makegetfctx(ctx)
 
     of = None
@@ -652,6 +653,10 b' def _checkcopies(ctx, f, m1, m2, base, t'
                 data['copy'][of] = f
             elif of in mb:
                 data['copy'][f] = of
+            elif remotebase: # special case: a <- b <- a -> b "ping-pong" rename
+                data['copy'][of] = f
+                del data['fullcopy'][f]
+                data['fullcopy'][of] = f
             else: # divergence w.r.t. graft CA on one side of topological CA
                 for sf in seen:
                     if sf in mb: