credential-cache--daemon: disallow relative socket path

Relative socket paths are dangerous since the user cannot generally
control when the daemon starts (initially, after a timeout, kill or
crash). Since the daemon creates but does not delete the socket
directory, this could lead to spurious directory creation relative
to the users cwd.
Suggested-by: NJeff King <peff@peff.net>
Signed-off-by: NJon Griffiths <jon_p_griffiths@yahoo.com>
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-credential-cache.txt

@@ -36,7 +36,7 @@ OPTIONS
 	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.
+	local filesystem. You must specify an absolute path.
 
 CONTROLLING THE DAEMON
 ----------------------

credential-cache--daemon.c

@@ -262,6 +262,9 @@ int main(int argc, const char **argv)
 	if (!socket_path)
 		usage_with_options(usage, options);
 
+	if (!is_absolute_path(socket_path))
+		die("socket directory must be an absolute path");
+
 	init_socket_directory(socket_path);
 	register_tempfile(&socket_file, socket_path);