Support old BSD find and "root dir symlink" better

Support old BSD find and "root dir symlink" better

* lisp/progmodes/grep.el (grep-compute-defaults):
Add '-H' to grep-find-template (bug#48471).

* lisp/cedet/semantic/symref/grep.el (semantic-symref-perform-search):
Pass the root directory name without the trailing slash.

* lisp/progmodes/xref.el (xref-matches-in-directory): Ditto.

* test/lisp/progmodes/xref-tests.el (xref--xref-file-name-display-is-abs)
(xref--xref-file-name-display-is-relative-to-project-root):
Make tests more strict again.