"git config --get" learned the "--default" option, to help the
calling script. Building on top of the tb/config-type topic, the
"git config" learns "--type=color" type. Taken together, you can
do things like "git config --get foo.color --default blue" and get
the ANSI color sequence for the color given to foo.color variable,
or "blue" if the variable does not exist.
* tb/config-default:
builtin/config: introduce `color` type specifier
config.c: introduce 'git_config_color' to parse ANSI colors
builtin/config: introduce `--default`