credential-cache: use XDG_CACHE_HOME for socket

Make git-credential-cache follow the XDG base path specification by
default. This increases consistency with other applications and helps
keep clutter out of users' home directories.

Check the old socket location, ~/.git-credential-cache/, and use
~/.git-credential-cache/socket if that directory exists rather than
forcing users who have used `git credential-cache` before to migrate to
the new XDG compliant location.
Otherwise use the socket $XDG_CACHE_HOME/git/credential/socket following
XDG base path specification. Use the subdirectory credential/ in case
other files are cached under $XDG_CACHE_HOME/git/ in the future and to
make the socket's purpose clear.

Signed-off-by: Devin Lehmacher <lehmacdj@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-credential-cache.txt

@@ -33,10 +33,13 @@ OPTIONS
 --socket <path>::
 
 	Use `<path>` to contact a running cache daemon (or start a new
-	cache daemon if one is not started). Defaults to
-	`~/.git-credential-cache/socket`. If your home directory is on a
-	network-mounted filesystem, you may need to change this to a
-	local filesystem. You must specify an absolute path.
+	cache daemon if one is not started).
+	Defaults to `$XDG_CACHE_HOME/git/credential/socket` unless
+	`~/.git-credential-cache/` exists in which case
+	`~/.git-credential-cache/socket` is used instead.
+	If your home directory is on a network-mounted filesystem, you
+	may need to change this to a local filesystem. You must specify
+	an absolute path.
 
 CONTROLLING THE DAEMON
 ----------------------

credential-cache.c

@@ -83,6 +83,19 @@ static void do_cache(const char *socket, const char *action, int timeout,
 	strbuf_release(&buf);
 }
 
+static char *get_socket_path(void)
+{
+	struct stat sb;
+	char *old_dir, *socket;
+	old_dir = expand_user_path("~/.git-credential-cache");
+	if (old_dir && !stat(old_dir, &sb) && S_ISDIR(sb.st_mode))
+		socket = xstrfmt("%s/socket", old_dir);
+	else
+		socket = xdg_cache_home("credential/socket");
+	free(old_dir);
+	return socket;
+}
+
 int cmd_main(int argc, const char **argv)
 {
 	char *socket_path = NULL;
@@ -106,7 +119,7 @@ int cmd_main(int argc, const char **argv)
 	op = argv[0];
 
 	if (!socket_path)
-		socket_path = expand_user_path("~/.git-credential-cache/socket");
+		socket_path = get_socket_path();
 	if (!socket_path)
 		die("unable to find a suitable socket path; use --socket");