log --show-signature: reword the common two-head merge case

log --show-signature: reword the common two-head merge case

While identifying the commit merged to our history as "parent #2" is
technically correct, we will never say "parent #1" (as that is the tip of
our history before the merge is made), and we rarely would say "parent #3"
(which would mean the merge is an octopus), especially when responding to
a request to pull a signed tag.

Treat the most common case to merge a single commit specially, and just
say "merged tag '<tagname>'" instead.

Signed-off-by: Junio C Hamano <gitster@pobox.com>