completion: zsh: stop leaking local cache variable

Completing commands like "git rebase" in one repository will leak the
local __git_repo_path into the shell's environment so that completing
commands after changing to a different repository will give the old
repository's references (or none at all).

The bug report on the mailing list [1] suggests one simple way to observe
this yourself:

Enter the following commands from some directory:
  mkdir a b b/c
  for d (a b); git -C $d init && git -C $d commit --allow-empty -m init
  cd a
  git branch foo
  pushd ../b/c
  git branch bar

Now type these:
  git rebase <TAB>… # completion for bar available; C-c to abort
  declare -p __git_repo_path # outputs /path/to/b/.git
  popd
  git branch # outputs foo, main
  git rebase <TAB>… # completion candidates are bar, main!

Ideally, the last typed <TAB> should be yielding foo, main.

Commit beb6ee7163 (completion: extract repository discovery from
__gitdir(), 2017-02-03) anticipated this problem by marking
__git_repo_path as local in __git_main and __gitk_main for Bash
completion but did not give the same mark to _git for Zsh completion.
Thus make __git_repo_path local for Zsh completion, too.

[1]: https://lore.kernel.org/git/CALnO6CBv3+e2WL6n6Mh7ZZHCX2Ni8GpvM4a-bQYxNqjmgZdwdg@mail.gmail.com/

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

contrib/completion/git-completion.zsh

@@ -272,6 +272,7 @@ _git ()
 {
 	local _ret=1
 	local cur cword prev
+	local __git_repo_path
 
 	cur=${words[CURRENT]}
 	prev=${words[CURRENT-1]}