git-worktree: Parse wrap files instead of meson introspect

git-worktree: Parse wrap files instead of meson introspect

meson introspect is the wrong approach since it:

* Requires a pre-existing build directory for some branch
* Gives us potentially incorrect information since it tells us
  subproject details for the current branch, not the branch we're
  checking out.
* Does not allow us to share the git repos for non-gst repos since it
  can't tell us the git branches for them.

Instead, parse the wrap files in the branch we're checking out since
they're just INI-style config files.