[BUG] "git merge-file" mismerge

Consider these three files:

    $ printf "%s\n" A B C D >ours
    $ printf "%s\n" A C B D >common
    $ printf "%s\n" A B D >theirs

Starting from A C B D, our side flips the order of the second (C)
and the third (B), while their side removes the second (C).

The correct three-way content level merge should notice conflict
during this merge, but "git merge-file" that uses xdiff/xmerge.c,
which is meant to be equivalent to "merge" from the RCS suite,
declares that our version is already the correct merge result:

    $ merge ours common theirs
    merge: warning: conflicts during merge
    $ cat ours
    A
    <<<<<<< ours
    B
    C
    =======
    B
    >>>>>>> theirs
    D
    $ printf "%s\n" A B C D >ours ;# revert it back
    $ git merge-file ours common theirs ; echo $?
    0
    $ cat ours
    A
    B
    C
    D