Proposed patch - Better choice of default for gtags-visit-rootdir

When in a file buffer that resides in what will be the gtags
root directory (because the GTAGS et al have not yet been created),
invoking gtags-visit-rootdir and pressing RETURN to accept the
default results in the file being chosen rather than the
(displayed) directory.

This is because read-file-name is using the absolute file name
of the file:

 > If both default and initial are nil and the buffer is visiting
 > a file, read-file-name uses the absolute file name of that
 > file as default.


--- gtags.el    2015-09-14 19:48:17.000000000 -0700
+++ ../share/gtags/gtags.el     2015-09-26 13:22:52.490185835 -0700
@@ -515,13 +515,17 @@
  (defun gtags-visit-rootdir ()
    "Tell tags commands the root directory of source tree."
    (interactive)
-  (let (path input)
+  (let (path input default-path)
      (setq path gtags-rootdir)
      (if (not path)
          (setq path (gtags-get-rootpath)))
      (if (not path)
          (setq insert-default-directory (if (string-match 
gtags-tramp-path-regexp default-directory) nil t)))
-    (setq input (read-file-name "Visit root directory: " path path t))
+    (setq default-path
+        (if (and insert-default-directory (not path))
+            default-directory
+            path))
+    (setq input (read-file-name "Visit root directory: " path 
default-path t))
      (if (equal "" input) nil
        (if (not (file-directory-p input))
          (message "%s is not directory." input)