merge-tree: remove redundant code

merge-tree: remove redundant code

real_merge() only ever returns "0" or "1" as it dies if the merge status
is less than zero. Therefore the check for "result < 0" is redundant and
the result variable is not needed. The return value of real_merge() is
ignored because exit status of "git merge-tree --stdin" is "0" for both
successful and conflicted merges (the status of each merge is written to
stdout). The return type of real_merge() is not changed as it is used
for the program's exit status when "--stdin" is not given.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>