Escape dash in xref rgrep regexp

* lisp/progmodes/xref.el (xref-collect-matches): Escape dash
in REGEXP if it's the first character.

lisp/progmodes/xref.el

@@ -918,6 +918,10 @@ IGNORES is a list of glob patterns."
   (grep-compute-defaults)
   (defvar grep-find-template)
   (defvar grep-highlight-matches)
+  ;; 'grep -E -foo' results in 'grep: oo: No such file or directory'.
+  ;; while 'grep -e -foo' inexplicably doesn't.
+  (when (eq (aref regexp 0) ?-)
+    (setq regexp (concat "\\" regexp)))
   (let* ((grep-find-template (replace-regexp-in-string "-e " "-E "
                                                        grep-find-template t t))
          (grep-highlight-matches nil)