duplicatecopies: do not mark items not in the dirstate as copies
Consider the following repo:
0 -- 1 (renames a to b) \ - 2
If we're rebasing 2 onto 1, then duplicatecopies is called with arguments (2,
1). copies.pathcopies goes backwards from 1 to 0 and returns the pair dst = a,
src = b. Of course, since we're working on top of 2, at this point a doesn't
exist in the dirstate.
Extra entries in the copymap are currently harmless because the copymap is
only queried for items in the dirstate map. However, if the dirstate.copy
method becomes one of the sources used to determine which files have changed,
this will prove problematic.
Note that we can't avoid going backwards in general -- consider this repo:
0 -- 1 (renames a to b) \ - 2 (renames a to c)
Rebasing 2 onto 1 should produce a rename from b to c.