completion: fix Zsh handling of stash names

When Git completes stashes for Zsh, it eventually invokes something like
this (via our redefinition of __gitcomp_nl):

    compadd -Q -S " " -p "" -- 'stash@{0}' 'stash@{1}' && _ret=0

Because of -Q, the braces are unquoted, and Zsh will leave us at (for
example)

    git stash show stash@{
                   cursor ^

>From here, the list-choices widget (on Ctrl-D by default) will show
stash@{0} and stash@{1}, but the complete-word widget (Tab) will just
add another brace. Zsh things we're trying to do a brace expansion, I
think! Interestingly, using menu-complete (Ctrl-N) does cycle through
matches, since it's taking a slightly different completion path in Zsh.

We might try to fix this by passing surrounding single quotes (using the
-S/-p arguments of compadd, which we can do via __gitcomp_nl's other
parameters), and indeed Zsh will generate

    git stash show 'stash@{
                    cursor ^

but completing here won't find anything, and I'm not totally sure why.

Instead, though, we can let Zsh quote metacharacters (removing -Q) and
everything "just works." It changes the output format a bit (users see
stash@\{0\}, for example), but I find this an improvement over not being
able to complete names at all. menu-complete is unchanged aside from the
extra backslashes.

So, override stash listing completion in Zsh with a working version.

Signed-off-by: D. Ben Knoble <ben.knoble@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.zsh

@@ -135,6 +135,11 @@ __gitcomp_file_direct ()
 	__gitcomp_file "$1" ""
 }
 
+__git_complete_stashes ()
+{
+	compadd -S ' ' -- ${(f)"$(__git stash list | sed -n -e 's/:.*//p')"} && _ret=0
+}
+
 _git_zsh ()
 {
 	__gitcomp "v1.1"