builtin/blame: dim uninteresting metadata lines

builtin/blame: dim uninteresting metadata lines

When using git-blame lots of lines contain redundant information, for
example in hunks that consist of multiple lines, the metadata (commit
name, author, date) are repeated. A reader may not be interested in those,
so offer an option to color the information that is repeated from the
previous line differently. Traditionally, we use CYAN for lines that
are less interesting than others (e.g. hunk header), so go with that.

The command line option '--color-lines' will trigger the coloring of
repeated lines, and the config option 'color.blame.colorLines' is
provided to select the color. Setting the config option doesn't imply
that repeated lines are colored. A later patch will introduce a config
to enable this mode by default.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>