projtool.pl: add worktree subcommand

projtool.pl: add worktree subcommand

Provide a worktree subcommand for projtool.pl to make it much
simpler to add a worktree to a project.

Simply pass the arguments along to the `git worktree` command
together with a --git-dir=<project-git-directory> option.

However, as a special case, if the project is bare (the usual
case) and its HEAD branch has not been checked out in any worktree
and no explicit branch name was given, just automagically DWIM and
supply the branch name when passing along the command.  And at the
same time make it work for an unborn branch too.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>