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.
error: pretxncommit.nocommits hook failed: no commits allowed
abort: no commits allowed
transaction abort!
rollback completed
error: pretxncommit.nocommits hook failed: no commits allowed
abort: no commits allowed
transaction abort!
rollback completed