bug#68377: Cannot commit parts of *vc-diff* on Microsoft Windows

[Sam Steingold]

https://emacs.stackexchange.com/q/80039/795

Emacs has a beautiful feature: in the `*vc-diff*` buffer (generated by 
[`vc-diff`](https://www.gnu.org/software/emacs/manual/html_node/emacs/Old-Revisions.html)
 either by <kbd>C-x v =</kbd> in a file buffer or by <kbd>=</kbd> in a 
`*vc-dir*` buffer), kill unwanted hunks with <kbd>k</kbd> (i.e., 
[`diff-hunk-kill`](https://www.gnu.org/software/emacs/manual/html_node/emacs/Diff-Mode.html))
 and then commit the remaining hunks with <kbd>C-x v v</kbd> (i.e., 
[`vc-next-action`](https://www.gnu.org/software/emacs/manual/html_node/emacs/Basic-VC-Editing.html)).
 If you don't know about it -- try it, it is incredibly useful!

Alas, it does _not_ work for me on windows: after <kbd>C-c C-c</kbd> (i.e., 
[`log-edit-done`](https://www.gnu.org/software/emacs/manual/html_node/emacs/Log-Buffer.html))
 in `*vc-log*`, I get 
> vc-do-command: Failed (status 1): 
> c:/Users/ssteingold/AppData/Local/Git/bin/git.exe --no-pager apply --cached 
> c:/msys64/tmp/git-patchPbbWgJ

and `*vc*` buffer is
```
error: patch failed: fxpy/model.py:1071
error: fxpy/model.py: patch does not apply
```

***Additionally***: 

1. Software:
   * [`GNU Emacs 29.1 (build 2, x86_64-w64-mingw32) of 
2023-08-02`](https://ftp.gnu.org/gnu/emacs/windows/emacs-29/)
   * [`git version 2.42.0.windows.2`](https://gitforwindows.org/)
1. *normal* commits work just **fine**, using `vc-next-action` from either file 
buffer or `*vc-dir*`.
1. All my files have unix line terminators (`LF`) as indicated in the mode line 
as `(Unix)`
1. I set `autocrlf` to `false` globally
1. I have `(prefer-coding-system 'utf-8-unix)` in `.emacs`
1. `vc-coding-system-inherit-eol` is `t`, I did not touch it
1. When I add `"-v"` to `git apply` invocation in `vc-git-checkin` 
(`vc-git.el`), `*vc*` buffer contains `error: while searching for:.....` and 
every line in `.....` is terminated with `?` - which seems to indicate 
line-ending problems... (!!!)

***Q***: how do I fix this?
(This is a work box, I cannot send email from Emacs, so I cannot even file an 
Emacs bug report!)

# work-around 1

Add 
```
(setq coding-system-for-write 'utf-8-unix)
```
inside `(with-temp-file patch-file ...)` in `vc-git-checkin`

# work-around 2

Add
```
;; replace `undecided`:
(setf (cadr (assoc "" file-coding-system-alist)) 'utf-8-unix)
```
to `.emacs`

# Emacs bug

`vc-git-checkin` should respect `vc-coding-system-inherit-eol` and set 
[`coding-system-for-write`](https://www.gnu.org/software/emacs/manual/html_node/elisp/Specifying-Coding-Systems.html)
 when writing the patch.

Also, it does not seem reasonable that 
[`prefer-coding-system`](https://www.gnu.org/software/emacs/manual/html_node/emacs/Output-Coding.html)
 does not affect 
[`file-coding-system-alist`](https://www.gnu.org/software/emacs/manual/html_node/elisp/Default-Coding-Systems.html).