checkcopies: add a sanity check against false-positive copies

When grafting a copy backwards through a rename, a copy is wrongly detected,
which causes the graft to be applied inappropriately, in a destructive way.
Make sure that the old file name really exists in the common ancestor,
and bail out if it doesn't.

This fixes the aggravated case of bug 5343, although the basic issue
(failure to duplicate the copy information) still occurs.

mercurial/copies.py

@@ -543,7 +543,8 b' def _checkcopies(ctx, f, m1, m2, base, l'
         # unrelated to the droids we are looking for.
         cr = _related(oc, c2, base.rev())
         if cr and (of == f or of == c2.path()): # non-divergent
-            data['copy'][f] = of
+            if of in mb:
+                data['copy'][f] = of
             return
 
     if of in mb:

tests/test-graft.t

@@ -427,8 +427,8 b' Graft with --log'
   $ hg graft 3 --log -u foo
   grafting 3:4c60f11aa304 "3"
   warning: can't find ancestor for 'c' copied from 'b'!
-  $ hg log --template '{rev} {parents} {desc}\n' -r tip
-  14 1:5d205f8b35b6  3
+  $ hg log --template '{rev}:{node|short} {parents} {desc}\n' -r tip
+  14:0c921c65ef1e 1:5d205f8b35b6  3
   (grafted from 4c60f11aa304a54ae1c199feb94e7fc771e51ed8)
 
 Resolve conflicted graft
@@ -620,7 +620,7 b' Test simple destination'
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     2
   
-  changeset:   14:f64defefacee
+  changeset:   14:0c921c65ef1e
   parent:      1:5d205f8b35b6
   user:        foo
   date:        Thu Jan 01 00:00:00 1970 +0000