want_color: automatically fallback to color.ui

All of the "do we want color" flags default to -1 to
indicate that we don't have any color configured. This value
is handled in one of two ways:

  1. In porcelain, we check early on whether the value is
     still -1 after reading the config, and set it to the
     value of color.ui (which defaults to 0).

  2. In plumbing, it stays untouched as -1, and want_color
     defaults it to off.

This works fine, but means that every porcelain has to check
and reassign its color flag. Now that want_color gives us a
place to put this check in a single spot, we can do that,
simplifying the calling code.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index 295803a..9763146 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1237,10 +1237,6 @@
 
 	if (s.relative_paths)
 		s.prefix = prefix;
-	if (s.use_color == -1)
-		s.use_color = git_use_color_default;
-	if (diff_use_color_default == -1)
-		diff_use_color_default = git_use_color_default;
 
 	switch (status_format) {
 	case STATUS_FORMAT_SHORT:
@@ -1394,15 +1390,10 @@
 	git_config(git_commit_config, &s);
 	determine_whence(&s);
 
-	if (s.use_color == -1)
-		s.use_color = git_use_color_default;
 	argc = parse_and_validate_options(argc, argv, builtin_commit_usage,
 					  prefix, &s);
-	if (dry_run) {
-		if (diff_use_color_default == -1)
-			diff_use_color_default = git_use_color_default;
+	if (dry_run)
 		return dry_run_commit(argc, argv, prefix, &s);
-	}
 	index_file = prepare_index(argc, argv, prefix, 0);
 
 	/* Set up everything for writing the commit object.  This includes