How to create a patch (git diff) with a specific name

Question

I would like to create a patch (git diff) with custom name -- e.g., hello-world.diff. Emacs doesn't seem to be able to handle git diff > hello-world.diff, perhaps because of the greater-than sign that works in regular terminals.

I have tried using start-process, but it fails:

(defun create-diff-patch ()
  (interactive)
    (let ((name-of-patch
           (read-string "Name of Patch:  " nil nil "patch.diff" nil)))
      (start-process "process-name" "*output-buffer*"
                     "git" "diff" ">" name-of-patch)))

Here is the error message:

fatal: ambiguous argument '>': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

Process process-name exited abnormally with code 128

The following two examples create automatic names, but I'd like to use my own name instead:

(apply #'magit-call-git
       (magit-process-quote-arguments '("format-patch" "-1" "SHA1")))

(apply #'magit-call-git
       (magit-process-quote-arguments '("format-patch" "HEAD~~")))

Answer 1

One possibility (not tested, but the principle should be sound) is something like this:

(defun create-diff-patch (filename)
  (start-process "git-diff" nil "/bin/sh" "-c"
         (format "git diff > %s" (shell-quote-argument filename))))

A more elaborate solution would be to use make-process to collect the git output in a temporary buffer, then use a process sentinel to save the buffer in the file you want when the process finishes.