builtin/blame: highlight recently changed lines

builtin/blame: highlight recently changed lines

Choose a different color for dates and imitate a 'temperature cool down'
depending upon age.

Originally I had planned to have the temperature cool down dependent on
the age of the project or file for example, as that might scale better,
but that can be added on top of this commit, e.g. instead of giving a
date, you could imagine giving a percentage that would be the linearly
interpolated between now and the beginning of the file.

Similarly to the previous patch, this offers the command line option
'--color-by-age' to enable this mode and the config option
'color.blame.highlightrecent' to select colors. A later patch will offer
a config option to select the default mode.

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