Merge branch 'cc/skip-to-optional-val'

Introduce a helper to simplify code to parse a common pattern that
expects either "--key" or "--key=<something>".

* cc/skip-to-optional-val:
  t4045: reindent to make helpers readable
  diff: add tests for --relative without optional prefix value
  diff: use skip_to_optional_arg_default() in parsing --relative
  diff: use skip_to_optional_arg_default()
  diff: use skip_to_optional_arg()
  index-pack: use skip_to_optional_arg()
  git-compat-util: introduce skip_to_optional_arg()

strbuf.c

@@ -11,6 +11,28 @@ int starts_with(const char *str, const char *prefix)
 			return 0;
 }
 
+int skip_to_optional_arg_default(const char *str, const char *prefix,
+				 const char **arg, const char *def)
+{
+	const char *p;
+
+	if (!skip_prefix(str, prefix, &p))
+		return 0;
+
+	if (!*p) {
+		if (arg)
+			*arg = def;
+		return 1;
+	}
+
+	if (*p != '=')
+		return 0;
+
+	if (arg)
+		*arg = p + 1;
+	return 1;
+}
+
 /*
  * Used as the default ->buf value, so that people can always assume
  * buf is non NULL and ->buf is NUL terminated even for a freshly