help: use parse_config_key for man config

The resulting code ends up about the same length, but it is
a little more self-explanatory. It now explicitly documents
and checks the pre-condition that the incoming var starts
with "man.", and drops the magic offset "4".

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/help.c

@@ -237,21 +237,21 @@ static int add_man_viewer_cmd(const char *name,
 
 static int add_man_viewer_info(const char *var, const char *value)
 {
-	const char *name = var + 4;
-	const char *subkey = strrchr(name, '.');
+	const char *name, *subkey;
+	int namelen;
 
-	if (!subkey)
+	if (parse_config_key(var, "man", &name, &namelen, &subkey) < 0 || !name)
 		return 0;
 
-	if (!strcmp(subkey, ".path")) {
+	if (!strcmp(subkey, "path")) {
 		if (!value)
 			return config_error_nonbool(var);
-		return add_man_viewer_path(name, subkey - name, value);
+		return add_man_viewer_path(name, namelen, value);
 	}
-	if (!strcmp(subkey, ".cmd")) {
+	if (!strcmp(subkey, "cmd")) {
 		if (!value)
 			return config_error_nonbool(var);
-		return add_man_viewer_cmd(name, subkey - name, value);
+		return add_man_viewer_cmd(name, namelen, value);
 	}
 
 	return 0;