config.c: introduce 'git_config_color' to parse ANSI colors

In preparation for adding `--type=color` to the `git-config(1)` builtin,
let's introduce a color parsing utility, `git_config_color` in a similar
fashion to `git_config_<type>`.
Signed-off-by: NTaylor Blau <me@ttaylorr.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

config.c

@@ -16,6 +16,7 @@
 #include "string-list.h"
 #include "utf8.h"
 #include "dir.h"
+#include "color.h"
 
 struct config_source {
 	struct config_source *prev;
@@ -1000,6 +1001,15 @@ int git_config_expiry_date(timestamp_t *timestamp, const char *var, const char *
 	return 0;
 }
 
+int git_config_color(char *dest, const char *var, const char *value)
+{
+	if (!value)
+		return config_error_nonbool(var);
+	if (color_parse(value, dest) < 0)
+		return -1;
+	return 0;
+}
+
 static int git_default_core_config(const char *var, const char *value)
 {
 	/* This needs a better name */

config.h

@@ -59,6 +59,7 @@ extern int git_config_bool(const char *, const char *);
 extern int git_config_string(const char **, const char *, const char *);
 extern int git_config_pathname(const char **, const char *, const char *);
 extern int git_config_expiry_date(timestamp_t *, const char *, const char *);
+extern int git_config_color(char *, const char *, const char *);
 extern int git_config_set_in_file_gently(const char *, const char *, const char *);
 extern void git_config_set_in_file(const char *, const char *, const char *);
 extern int git_config_set_gently(const char *, const char *);