shortlog: make trailer insertion a noop when appropriate

shortlog: make trailer insertion a noop when appropriate

When there are no trailers to insert, it is natural that
insert_records_from_trailers() should return without having done any
work.

But instead we guard this call unnecessarily by first checking whether
`log->groups` has the `SHORTLOG_GROUP_TRAILER` bit set.

Prepare to match a similar pattern in the future where a function which
inserts records of a certain type does no work when no specifiers
matching that type are given.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>