Force various files to be skipped when generating git archives

Force various files to be skipped when generating git archives

Meson utilizes `git archive` as the first step in producing a release
tarball. Certain files don't need to be distributed in release tarballs
at all -- namely, files used by git itself, or by github when processing
a git repository for e.g. CI purposes. They have no use case outside of
a git repo -- not even as documentation.

Exclude these files from the tarball. This affects both `meson dist` and
the github autogenerated tarballs on the Tags page. More work is needed
to have `meson dist` reach parity with automake's `make distcheck`, but
this is a start.