use real location when hashing.

Fix following issue:

Step 1. In a non-GTAGSLIBPATH path /path/to/src, call helm-gtags-select so helm-gtags-tag-location is set to /path/to/src.

Step 2. Switch to a GTAGSLIBPATH path /path/to/lib, call helm-gtags-select again but it shows the tags of /path/to/src because (helm-gtags-find-tag-directory) returns /path/to/src.