diff-line-info.el

(defun diff-line-info-as-kill (&optional prefix)
  "Copy a chunk of text with the file and line number of the diff line at point.

If the diff line is a deletion, this will be the path and line
number from the old file with (old) appended, otherwise the path
and line number of the new file.

With a prefix argument, remove that meaning leading elements from
the path.  The default is 1 (since that's mostly commonly
desired)."
  (interactive "P")
  (let* ((rev (save-excursion (beginning-of-line) (looking-at "[-<]")))
         (fname (remove-file-name-prefix (if (null prefix) 1 prefix)
                                         (car (diff-hunk-file-names rev)))))
    (set-text-properties 0 (length fname) nil fname)
    (kill-new (format "%s:%d%s" fname
                      (diff-find-source-offset nil rev)
                      (if rev " (old)" "")))))

;; Largely stolen from `diff-find-source-location'
(defun diff-find-source-offset (other-file rev)
  (save-excursion
    (let* ((char-offset (- (point) (diff-beginning-of-hunk t)))
          (line (split-string
                 (if (not (looking-at "\\(?:\\*\\{15\\}.*\n\\)?[-@* ]*\\([0-9,]+\\)\\([ acd+]+\\([0-9,]+\\)\\)?"))
                    (error "Can't find the hunk header")
                  (if rev (match-string 1)
                    (if (match-end 3) (match-string 3)
                      (unless (re-search-forward
                               diff-context-mid-hunk-header-re nil t)
                        (error "Can't find the hunk separator"))
                      (match-string 1)))) ","))
          (hunk (buffer-substring (point) (save-excursion (diff-end-of-hunk)
                                                          (point))))
          (old (diff-hunk-text hunk (not rev) char-offset))
          (new (diff-hunk-text hunk (not rev) char-offset))
          (line-offset
           (length (replace-regexp-in-string "[^\n]" ""
                                             (substring (car (if rev old new)) 0
                                                        (cdr (if rev old new)))))))
      (+ (string-to-number (car line)) line-offset))))

(defun remove-file-name-prefix (n path)
  (let ((path (split-string path "/" t)))
    (mapconcat #'identity (last path (- (length path) n)) "/")))