git-am: minor cleanup
git-am: minor cleanup

This moves the assignment to FIRSTLINE down so that we do not have
to have multiple copies.

Suggested by Linus.

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