completion: add new zsh completion

It seems there's always issues with zsh's bash completion emulation.
I've tried to fix as many as I could[1], and most of the fixes are already
in the latest version of zsh, but still, there are issues.

There is no point going through all that pain; the emulation is easy to
achieve, and this patch works better than zsh's bash completion
emulation.

[1] http://zsh.git.sourceforge.net/git/gitweb.cgi?p=zsh/zsh;a=commitdiff;h=23907bb840c80eef99eabba17e086e44c9b2d3fc

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.zsh

@@ -0,0 +1,78 @@
+#compdef git gitk
+
+# zsh completion wrapper for git
+#
+# You need git's bash completion script installed somewhere, by default on the
+# same directory as this script.
+#
+# If your script is on ~/.git-completion.sh instead, you can configure it on
+# your ~/.zshrc:
+#
+#  zstyle ':completion:*:*:git:*' script ~/.git-completion.sh
+#
+# The recommended way to install this script is to copy to
+# '~/.zsh/completion/_git', and then add the following to your ~/.zshrc file:
+#
+#  fpath=(~/.zsh/completion $fpath)
+
+complete ()
+{
+	# do nothing
+	return 0
+}
+
+zstyle -s ":completion:*:*:git:*" script script
+test -z "$script" && script="$(dirname ${funcsourcetrace[1]%:*})"/git-completion.bash
+ZSH_VERSION='' . "$script"
+
+__gitcomp ()
+{
+	emulate -L zsh
+
+	local cur_="${3-$cur}"
+
+	case "$cur_" in
+	--*=)
+		;;
+	*)
+		local c IFS=$' \t\n'
+		local -a array
+		for c in ${=1}; do
+			c="$c${4-}"
+			case $c in
+			--*=*|*.) ;;
+			*) c="$c " ;;
+			esac
+			array+=("$c")
+		done
+		compset -P '*[=:]'
+		compadd -Q -S '' -p "${2-}" -a -- array && _ret=0
+		;;
+	esac
+}
+
+__gitcomp_nl ()
+{
+	emulate -L zsh
+
+	local IFS=$'\n'
+	compset -P '*[=:]'
+	compadd -Q -S "${4- }" -p "${2-}" -- ${=1} && _ret=0
+}
+
+_git ()
+{
+	local _ret=1
+	() {
+		emulate -L ksh
+		local cur cword prev
+		cur=${words[CURRENT-1]}
+		prev=${words[CURRENT-2]}
+		let cword=CURRENT-1
+		__${service}_main
+	}
+	let _ret && _default -S '' && _ret=0
+	return _ret
+}
+
+_git