auth: have cgit calculate login address

This way we're sure to use virtual root, or any other strangeness
encountered.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

ui-shared.h

@@ -4,6 +4,7 @@
 extern const char *cgit_httpscheme();
 extern const char *cgit_hosturl();
 extern const char *cgit_rooturl();
+extern const char *cgit_loginurl();
 extern char *cgit_repourl(const char *reponame);
 extern char *cgit_fileurl(const char *reponame, const char *pagename,
 			  const char *filename, const char *query);