Enable color support based on tty and config

Use the git config command and provided options to have git detect
whether color support should be enabled or disabled. Additionally ensure
that git-review color support can be controlled separately via the
option 'color.review', while still falling back to 'color.ui' if the app
specific option is not defined.

This will ensure that when piping or redirecting the output that colour
output will be disabled unless color.review, or it's fallback color.ui,
is set to always.

Also by utilizing 'git config', the any config options specified on the
command line that change behaviour will be adhered to. e.g.

  git -c color.review=never review -l

Will always output without color enabled.

Change-Id: I8f83ed8623da88e87972109af956331704e15d38
Closes-Bug: #1097961
Merge "Enable color support based on tty and config"