builtin-fmt-merge-msg: fix bugs in --file option
builtin-fmt-merge-msg: fix bugs in --file option

If --file's argument is missing, don't crash.  If it cannot be opened,
die with an error message.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

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