log: add %S option (like --source) to log --format

log: add %S option (like --source) to log --format

Make it possible to write for example

        git log --format="%H,%S"

where the %S at the end is a new placeholder that prints out the ref
(tag/branch) for each commit.

Using %d might seem like an alternative but it only shows the ref for the last
commit in the branch.

Signed-off-by: Issac Trotts <issactrotts@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>