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>