completion: support git-worktree

This adds bare-bone completion support for git-worktree. More advanced
completion (e.g. ref completion in git-worktree-add) can be added later.

--force completion in "worktree add" is left out because that option
should be handled with care.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2595,6 +2595,29 @@ _git_whatchanged ()
 	_git_log
 }
 
+_git_worktree ()
+{
+	local subcommands="add list prune"
+	local subcommand="$(__git_find_on_cmdline "$subcommands")"
+	if [ -z "$subcommand" ]; then
+		__gitcomp "$subcommands"
+	else
+		case "$subcommand,$cur" in
+		add,--*)
+			__gitcomp "--detach"
+			;;
+		list,--*)
+			__gitcomp "--porcelain"
+			;;
+		prune,--*)
+			__gitcomp "--dry-run --expire --verbose"
+			;;
+		*)
+			;;
+		esac
+	fi
+}
+
 __git_main ()
 {
 	local i c=1 command __git_dir