bug#60126: 30.0.50; vc-git-checkin: Offer to unstage conflicting changes

[Eli Zaretskii]

> Cc: 60126@debbugs.gnu.org, juri@linkov.net
> From: Sean Whitton <spwhitton@spwhitton.name>
> Date: Thu, 22 Dec 2022 20:59:53 -0700
> 
> +        ;; Prepare stash commit object, which has a special structure.
> +        (let* ((tree-commit (git-string "commit-tree" "-m" message

Can 'message' include newlines? if so, we need to either convert the
newlines to spaces, or invoke the command with -F instead, writing the
message to a temporary file.  Because otherwise this will fail on
Windows.