commit: don't start editor if empty message is given with -m

commit: don't start editor if empty message is given with -m

If an empty message is specified with the option -m of git commit then
the editor is started.  That's unexpected and unnecessary.  Instead of
using the length of the message string for checking if the user
specified one, directly remember if the option -m was given.

Reported-by: Mislav Marohnić <redacted>
Signed-off-by: René Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>