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>

t/t3903-stash.sh

@@ -859,7 +859,7 @@ test_expect_success 'setup stash with index and worktree changes' '
 	git stash
 '
 
-test_expect_success 'stash list implies --first-parent -m' '
+test_expect_success 'stash list -p shows simple diff' '
 	cat >expect <<-EOF &&
 	stash@{0}