merge: fix a copy detection bug (issue672) When merging rev1 and rev2, we want to search for copies that happened in rev1 but not in rev2 and vice-versa. We were starting the search at rev1/rev2 and then going back, stopping as soon as we reached the revno of the ancestor, but that can miss some cases (see the new test-issue672). Now we calculate the revisions that are ancestors of rev1 or rev2 (but not both) and make sure the search doesn't stop too early. Simplified test provided by mpm, based on a test case provided by Edward Lee.

resolving manifests
 overwrite None partial False
 ancestor 310fd17130da local 2092631ce82b+ remote 7731dad1c2b9
 foo: versions differ -> m
 foo: remote copied to bar -> m
copying foo to bar
merging foo and bar
my foo@2092631ce82b+ other bar@7731dad1c2b9 ancestor foo@310fd17130da
merging foo
my foo@2092631ce82b+ other foo@7731dad1c2b9 ancestor foo@310fd17130da
0 files updated, 2 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
-- foo --
line 0
line 1
line 2-1
-- bar --
line 0
line 1
line 2-2