commit/status: check $GIT_DIR/MERGE_HEAD only once

commit/status: check $GIT_DIR/MERGE_HEAD only once

The code checked for the MERGE_HEAD file to see if we were about
to commit a merge twice in the codepath; also one of them used a
variable merge_head_sha1[] which was set but was never used.

Just check it once, but do so also in "git status", too, as
we will be using this for status generation in the next patch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>