t6023: merge-file fails to output anything for a degenerate merge
t6023: merge-file fails to output anything for a degenerate merge

In the case that merge-file is passed three files with identical
contents it wipes the contents of the output file instead of
leaving it unchanged.

Althought merge-file is porcelain and this will never happen in
normal usage, it is still wrong.

Signed-off-by: Charles Bailey <cbdb0cc7f3f5b4be81a75fa7242590e3e9882e1e@hashpling.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git