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.

adding foo
Patch queue now empty
applying patch1
applying patch2
applying bad-patch
transaction abort!
rollback completed
cleaning up working directory...done
abort: decoding near 'é': 'ascii' codec can't decode byte 0xe9 in position 0: ordinal not in range(128)!
changeset:   0:bbd179dfa0a7
tag:         tip
user:        test
date:        Thu Jan 01 00:00:00 1970 +0000
summary:     add foo

% bar should be gone; other unknown/ignored files should still be around
? untracked-file
I .hgignore
C foo