Restore <D> instead of '.' in grep-find-template

Restore <D> instead of '.' in grep-find-template

* lisp/cedet/semantic/symref/grep.el
(semantic-symref-grep-use-template): Update a comment.

* lisp/progmodes/grep.el (grep-compute-defaults): Restore <D>
instead of '.' in grep-find-template (bug#20719).
(rgrep): Pass nil as the directory to rgrep-default-command.

* lisp/progmodes/grep.el (grep-expand-keywords): Use '.' as the
default value for DIR.

* lisp/progmodes/xref.el (xref-collect-matches): Drop the
workaround.