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

color.py

@@ -83,15 +83,38 @@ def _Color(fg = None, bg = None, attr = None):
   return code
 
 
+DEFAULT = None
+
+def SetDefaultColoring(state):
+  """Set coloring behavior to |state|.
+
+  This is useful for overriding config options via the command line.
+  """
+  if state is None:
+    # Leave it alone -- return quick!
+    return
+
+  global DEFAULT
+  state = state.lower()
+  if state in ('auto',):
+    DEFAULT = state
+  elif state in ('always', 'yes', 'true', True):
+    DEFAULT = 'always'
+  elif state in ('never', 'no', 'false', False):
+    DEFAULT = 'never'
+
+
 class Coloring(object):
   def __init__(self, config, section_type):
     self._section = 'color.%s' % section_type
     self._config = config
     self._out = sys.stdout
 
-    on = self._config.GetString(self._section)
+    on = DEFAULT
     if on is None:
-      on = self._config.GetString('color.ui')
+      on = self._config.GetString(self._section)
+      if on is None:
+        on = self._config.GetString('color.ui')
 
     if on == 'auto':
       if pager.active or os.isatty(1):