+(defun git-diff-file-merge-head (arg)
+ "Diff the marked file(s) against the first merge head (or the nth one with a numeric prefix)."
+ (interactive "p")
+ (let ((files (git-marked-files))
+ (merge-heads (git-get-merge-heads)))
+ (unless merge-heads (error "No merge in progress"))
+ (git-setup-diff-buffer
+ (apply #'git-run-command-buffer "*git-diff*" "diff-index" "-p" "-M"
+ (or (nth (1- arg) merge-heads) "HEAD") "--" (git-get-filenames files)))))
+