completion: zsh: support completion after "git -C <path>"

The zsh completion wrapper (__git_zsh_main) did not handle the global -C
option, so "git -C <path> <command> <TAB>" offered nothing and could not
complete a command's arguments.

Three things are needed to make it work, all scoped to -C:

  - Add -C to the _arguments specification, so completion no longer stops
    at it.

  - Advance __git_cmd_idx past any leading "-C <path>" options. The index
    is hard-coded to 1, i.e. the command is assumed to be the first
    argument; with -C present the command sits two words later for each
    -C, so the bash helpers otherwise look at the wrong word and produce
    nothing.

  - Collect the -C paths into __git_C_args, as __git_main does. The bash
    helpers run git to resolve aliases and list refs; without the -C
    paths they run in the current directory, so completion fails whenever
    the cwd is not the target repository or the command is an alias.

With these, "git -C <path> <command> <TAB>" completes the command, its
options and its arguments, including outside the repository, through
aliases, and with repeated -C options.

Signed-off-by: Lutz Lengemann <lutz@lengemann.net>

Author: mobilutz

contrib/completion/git-completion.zsh

@@ -227,6 +227,7 @@ __git_zsh_main ()
 		'(-p --paginate --no-pager)'{-p,--paginate}'[pipe all output into ''less'']' \
 		'(-p --paginate)--no-pager[do not pipe git output into a pager]' \
 		'--git-dir=-[set the path to the repository]: :_directories' \
+		'*-C[run as if git was started in <path>]: :_directories' \
 		'--bare[treat the repository as a bare repository]' \
 		'(- :)--version[prints the git suite version]' \
 		'--exec-path=-[path to where your core git programs are installed]:: :_directories' \
@@ -252,6 +253,14 @@ __git_zsh_main ()
 		;;
 	(arg)
 		local command="${words[1]}" __git_dir __git_cmd_idx=1
+		local -a __git_C_args
+		local -i i=2
+
+		while [[ ${orig_words[i]} == -C ]]; do
+			__git_C_args+=(-C ${orig_words[i+1]})
+			(( __git_cmd_idx += 2 ))
+			(( i += 2 ))
+		done
 
 		if (( $+opt_args[--bare] )); then
 			__git_dir='.'