Add stgit.new.verbose config option

Add stgit.new.verbose config option

Instead of piggybacking on Git's commit.new config option, StGit now has
its own config option for turning-on `stg new --verbose`.

Two considerations for this change:

- `stg new` does not refresh, thus it leaves changes in the worktree.
  This is a significant semantic difference with `git commit` that makes
  reuse of `commit.verbose` an awkward fit.

- The performance implications in large monorepos with `stg new
  --verbose` versus `git commit --verbose` are quite different. See
  #145. The gist is that the diff operation needed for `stg new
  --verbose` can be much more expensive than `git commit --verbose`.

Signed-off-by: Peter Grayson <pete@jpgrayson.net>