stash list: stop passing "-m" to "git log"

stash list: stop passing "-m" to "git log"

Passing "-m" in "git log --first-parent -m" is not needed as
--first-parent implies --diff-merges=first-parent anyway. OTOH, it
will stop being harmless once we let "-m" imply "-p".

While we are at it, fix corresponding test description in t3903-stash
to match what it actually tests.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>