config: extract function to parse config pairs

The function `git_config_parse_parameter` is responsible for parsing a
`foo.bar=baz`-formatted configuration key, sanitizing the key and then
processing it via the given callback function. Given that we're about to
add a second user which is going to process keys which already has keys
and values separated, this commit extracts a function
`config_parse_pair` which only does the sanitization and processing
part as a preparatory step.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -462,11 +462,26 @@ int git_config_key_is_valid(const char *key)
 	return !git_config_parse_key_1(key, NULL, NULL, 1);
 }
 
+static int config_parse_pair(const char *key, const char *value,
+			  config_fn_t fn, void *data)
+{
+	char *canonical_name;
+	int ret;
+
+	if (!strlen(key))
+		return error(_("empty config key"));
+	if (git_config_parse_key(key, &canonical_name, NULL))
+		return -1;
+
+	ret = (fn(canonical_name, value, data) < 0) ? -1 : 0;
+	free(canonical_name);
+	return ret;
+}
+
 int git_config_parse_parameter(const char *text,
 			       config_fn_t fn, void *data)
 {
 	const char *value;
-	char *canonical_name;
 	struct strbuf **pair;
 	int ret;
 
@@ -487,12 +502,7 @@ int git_config_parse_parameter(const char *text,
 		return error(_("bogus config parameter: %s"), text);
 	}
 
-	if (git_config_parse_key(pair[0]->buf, &canonical_name, NULL)) {
+	ret = config_parse_pair(pair[0]->buf, value, fn, data);
-		ret = -1;
-	} else {
-		ret = (fn(canonical_name, value, data) < 0) ? -1 : 0;
-		free(canonical_name);
-	}
 	strbuf_list_free(pair);
 	return ret;
 }