add a global --color option

If you want to turn off colors for commands, you have to manually adjust
the git config settings (in various locations).  If you're writing scripts
though, you often don't want to modify those locations.  Add a commandline
option to explicitly control things.

The default behavior is unchanged -- we still scan the config files.

Change-Id: I54a3fd8e1918bac180aadd7c7d3004f069b02522

main.py

@@ -36,6 +36,7 @@ try:
 except ImportError:
   kerberos = None
 
+from color import SetDefaultColoring
 from trace import SetTrace
 from git_command import git, GitCommand
 from git_config import init_ssh, close_ssh
@@ -69,6 +70,9 @@ global_options.add_option('-p', '--paginate',
 global_options.add_option('--no-pager',
                           dest='no_pager', action='store_true',
                           help='disable the pager')
+global_options.add_option('--color',
+                          choices=('auto', 'always', 'never'), default=None,
+                          help='control color usage: auto, always, never')
 global_options.add_option('--trace',
                           dest='trace', action='store_true',
                           help='trace git command execution')
@@ -113,6 +117,8 @@ class _Repo(object):
         print('fatal: invalid usage of --version', file=sys.stderr)
         return 1
 
+    SetDefaultColoring(gopts.color)
+
     try:
       cmd = self.commands[name]
     except KeyError: