Add color support for colorizing more parts of diff output

[mbsulliv]

Can you add color support for colorizing the following additional parts of the diff output:

• Lines containing only ---, which separate commit log output from diff-stat output
• Diff lines beginning with --- a/
• Diff lines beginning with +++ b/
• Diff lines beginning with " " (a space, without the quotes), which are context lines in the diff output

I built from d59840e which is the HEAD on master at the moment.

[jonas]

With the referenced commit you can now add color rules such as in your ~/.tigrc:

color "--- a/" <fg> <bg>
color "--- b/" <fg> <bg>
color "---" <fg> <bg>
color " " <fg> <bg>