blame: document --color-* options

blame: document --color-* options

Commit cdc2d5f11f1a (builtin/blame: dim uninteresting metadata lines,
2018-04-23) and 25d5f52901f0 (builtin/blame: highlight recently changed
lines, 2018-04-23) introduce --color-lines and --color-by-age options to
git blame, respectively. While both options are mentioned in usage help,
they aren't documented in git-blame(1). Document them.

Co-authored-by: Dr. Matthias St. Pierre <m.st.pierre@ncp-e.com>
Signed-off-by: Dr. Matthias St. Pierre <m.st.pierre@ncp-e.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>