completion: correct zsh detection when run from git-completion.zsh

v2.18.0-rc0~90^2 (completion: reduce overhead of clearing cached
--options, 2018-04-18) worked around a bug in bash's "set" builtin on
MacOS by using compgen instead.  It was careful to avoid breaking zsh
by guarding this workaround with

	if [[ -n ${ZSH_VERSION-}} ]]

Alas, this interacts poorly with git-completion.zsh's bash emulation:

	ZSH_VERSION='' . "$script"

Correct it by instead using a new GIT_SOURCING_ZSH_COMPLETION shell
variable to detect whether git-completion.bash is being sourced from
git-completion.zsh.  This way, the zsh variant is used both when run
from zsh directly and when run via git-completion.zsh.

Reproduction recipe:

 1. cd git/contrib/completion && cp git-completion.zsh _git
 2. Put the following in a new ~/.zshrc file:

 	autoload -U compinit; compinit
	autoload -U bashcompinit; bashcompinit
	fpath=(~/src/git/contrib/completion $fpath)

 3. Open zsh and "git <TAB>".

With this patch:
Triggers nice git-completion.bash based tab completion

Without:
 contrib/completion/git-completion.bash:354: read-only variable: QISUFFIX
 zsh:12: command not found: ___main
 zsh:15: _default: function definition file not found
 _dispatch:70: bad math expression: operand expected at `/usr/bin/g...'
 Segmentation fault

Reported-by: Rick van Hattem <wolph@wol.ph>
Reported-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -3134,7 +3134,10 @@ __gitk_main ()
 	__git_complete_revlist
 }
-if [[ -n ${ZSH_VERSION-} ]]; then
+if [[ -n ${ZSH_VERSION-} ]] &&
+   # Don't define these functions when sourced from 'git-completion.zsh',
+   # it has its own implementations.
+   [[ -z ${GIT_SOURCING_ZSH_COMPLETION-} ]]; then
 	echo "WARNING: this script is deprecated, please see git-completion.zsh" 1>&2
 	autoload -U +X compinit && compinit

contrib/completion/git-completion.zsh

@@ -39,7 +39,7 @@ if [ -z "$script" ]; then
 		test -f $e && script="$e" && break
 	done
 fi
-ZSH_VERSION='' . "$script"
+GIT_SOURCING_ZSH_COMPLETION=y . "$script"
 __gitcomp ()
 {