It's pretty common to have the same branch for a subproject in
multiple worktrees. F.ex., when you want to test a feature branch
common to a few gstreamer subprojects but not the rest.
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.
This is useful to check that a build didn't result in changes in the
code/generated files
This will be used to check that the plugins documentation cache file is
properly commited, and that necessary workaround for particular case
are adopted.
Don't assume that meson is always a python script, on Windows it can
be (and soon will almost always be) an executable.
See: Meson MSI installer and https://github.com/mesonbuild/meson/pull/4004
