Avoid duplicate entries in `git ls-files` for unmerged entries

[dscho]

This is a left-over bit: https://public-inbox.org/git/xmqq8sw17c3q.fsf@gitster-ct.c.googlers.com/

[adlternative]

Hi ! I seem to fix this problem!
Look at:#832
Thanks!

[newren]

Already fixed in #832.