Pass color option to git diff (on failure)

Fixes #1007

pre_commit/commands/run.py

@@ -224,7 +224,14 @@ def _run_hooks(config, hooks, args, environ):
                 '`pre-commit install`.',
             )
         output.write_line('All changes made by hooks:')
-        subprocess.call(('git', '--no-pager', 'diff', '--no-ext-diff'))
+        if args.color:
+            subprocess.call((
+                'git', '--no-pager', 'diff', '--no-ext-diff',
+                '--color={}'.format(args.color),
+            ))
+        else:
+            subprocess.call(('git', '--no-pager', 'diff', '--no-ext-diff'))
+
     return retval
 
 

tests/commands/run_test.py

@@ -187,6 +187,13 @@ def test_global_exclude(cap_out, store, tempdir_factory):
             },
             b'All changes made by hooks:',
         ),
+        (
+            {
+                'show_diff_on_failure': True,
+                'color': 'auto',
+            },
+            b'All changes made by hooks:',
+        ),
         (
             {
                 'show_diff_on_failure': True,