Add start point to worktree add subcommand

worktree/_worktree

@@ -9,6 +9,14 @@ __worktree_branches() {
   _describe 'branch' branches
 }
 
+__worktree_start_points() {
+  local -a refs
+  refs=(
+    ${(fo)"$(git for-each-ref --format='%(refname:short)' refs/heads refs/remotes 2>/dev/null)"}
+  )
+  _describe 'start point' refs
+}
+
 __worktree_active_branches() {
   local -a branches
   branches=(${(fo)"$(git worktree list --porcelain 2>/dev/null | \
@@ -49,7 +57,9 @@ _worktree() {
       curcontext="${curcontext%:*:*}:worktree-cmd-$words[1]:"
       case $line[1] in
         (add)
-          _arguments '1:: :__worktree_branches'
+          _arguments \
+            '1:: :__worktree_branches' \
+            '2:: :__worktree_start_points'
           ;;
         (cd)
           _arguments '1:: :__worktree_all_branches'