config: refactor get_colorbool function

For "git config --get-colorbool color.foo", we use a custom
callback that looks not only for the key that the user gave
us, but also for "diff.color" (for backwards compatibility)
and "color.ui" (as a fallback).

For the former, we use a custom variable to store the
diff.color value. For the latter, though, we store it in the
main "git_use_color_default" variable, turning on color.ui
for any other parts of git that respect this value.

In practice, this doesn't cause any bugs, because git-config
runs without caring about git_use_color_default, and then
exits. But it crosses module boundaries in an unusual and
confusing way, and it makes refactoring color handling
harder than it needs to be.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/config.c

@@ -305,6 +305,7 @@ static void get_color(const char *def_color)
 
 static int get_colorbool_found;
 static int get_diff_color_found;
+static int get_color_ui_found;
 static int git_get_colorbool_config(const char *var, const char *value,
 		void *cb)
 {
@@ -313,7 +314,7 @@ static int git_get_colorbool_config(const char *var, const char *value,
 	else if (!strcmp(var, "diff.color"))
 		get_diff_color_found = git_config_colorbool(var, value);
 	else if (!strcmp(var, "color.ui"))
-		git_use_color_default = git_config_colorbool(var, value);
+		get_color_ui_found = git_config_colorbool(var, value);
 	return 0;
 }
 
@@ -327,7 +328,7 @@ static int get_colorbool(int print)
 		if (!strcmp(get_colorbool_slot, "color.diff"))
 			get_colorbool_found = get_diff_color_found;
 		if (get_colorbool_found < 0)
-			get_colorbool_found = git_use_color_default;
+			get_colorbool_found = get_color_ui_found;
 	}
 
 	get_colorbool_found = want_color(get_colorbool_found);