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 baradding foo% state dumpa 644 0 bazn 644 0 foor 0 0 bar% statusA bazR barC foo% state dumpn 666 -1 barn 666 -1 foo% status! bar? bazC foo