Add worktree sync subcommand

This `worktree sync` subcommand follows on from removing symbolic links
from `worktree add` to update all worktrees which don't have the latest
version of shared files from the root worktree.

worktree/_worktree

@@ -32,6 +32,7 @@ _worktree() {
         'ls:List managed worktrees'
         'remove:Remove a worktree by branch'
         'rm:Remove a worktree by branch'
+        'sync:Sync copied files from root to worktrees'
       )
       _describe -t commands 'worktree command' commands "$@"
       ;;
@@ -44,6 +45,9 @@ _worktree() {
         (rm|remove)
           _arguments '*:: :__worktree_active_branches'
           ;;
+        (sync)
+          _arguments '*:: :__worktree_active_branches'
+          ;;
       esac
       ;;
   esac