convert_git: avoid returning two entries for the same file in getchanges
This could happen in merge changesets if the merged file was different
from both parents.
adding dir/bar
adding foo
dir/bar
foo
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2
adding a
adding c
adding b
adding d
removing a
removing c
recording removal of a as rename to b (100% similar)