wt-add: Allow creation of new branches

[keggsmurph21]

Current behavior:

$ pwd
/path/to/master
$ git branch --list
* master
$ wt add new-branch
worktree-add returned code 128

fatal: invalid reference: z

$ git branch --list
* master
$ pwd
/path/to/master

Desired behavior:

$ pwd
/path/to/master
$ git branch --list
* master
$ wt add new-branch
$ git branch --list
  master
* new-branch
$ pwd
~/.worktrees/:owner/:repo/new-branch