grep: Only use --color on git 1.6.3 and later

The --color flag wasn't introduced until git 1.6.3.  Prior to that
version, `git grep --color` just produces a fatal error, as it is
an unsupported option.  Since this is just pretty output and is not
critical to execution, we can simply omit the option if the version
of git we are running on doesn't support it.

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/grep.py

@@ -17,7 +17,7 @@ import sys
 from optparse import SUPPRESS_HELP
 from color import Coloring
 from command import PagedCommand
-from git_command import GitCommand
+from git_command import git_require, GitCommand
 
 class GrepColoring(Coloring):
   def __init__(self, config):
@@ -158,7 +158,7 @@ contain a line that matches both expressions:
     out = GrepColoring(self.manifest.manifestProject.config)
 
     cmd_argv = ['grep']
-    if out.is_on:
+    if out.is_on and git_require((1,6,3)):
       cmd_argv.append('--color')
     cmd_argv.extend(getattr(opt,'cmd_argv',[]))