M-x rgrep, but using git grep from the top-level git dir

git-grep.el

;; M-x rgrep, but using git grep from the top-level git dir.
;; Based on http://stackoverflow.com/questions/25633490/how-can-i-use-m-x-rgrep-with-the-git-grep-command-in-emacs
(defcustom git-grep-command "git --no-pager grep --no-color --line-number <C> <R> | sed 's|^|./|'"
  "The command to run with M-x git-grep.")
(defun git-grep (regexp)
  "Search for the given regexp using `git grep' in the current directory."
  (interactive "sRegexp: ")
  (unless (boundp 'grep-find-template) (grep-compute-defaults))
  (let ((old-command grep-find-template))
    (grep-apply-setting 'grep-find-template git-grep-command)
    (rgrep regexp "*" (car (process-lines "git" "rev-parse" "--show-toplevel")))
    (grep-apply-setting 'grep-find-template old-command)))