git-log: allow --decorate[=short|full]

git-log: allow --decorate[=short|full]

Commit de435ac0 changed the behavior of --decorate from printing the
full ref (e.g., "refs/heads/master") to a shorter, more human-readable
version (e.g., just "master"). While this is nice for human readers,
external tools using the output from "git log" may prefer the full
version.

This patch introduces an extension to --decorate to allow the caller to
specify either the short or the full versions.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>