tests/embedded: improve version extraction

Avoid troubles when the tarball name contains another instance of 'tar'.
Update .gitignore.
Convert SVN URLs to git URLs in documentation.
Update to only handle Git repos.

No longer handle SVN or Mercurial repos.
Updates for tarball generation to handle new git infrastructure.

- No longer download latest config.sub|guess in distscript.csh; the
  GNU Autotools are updated frequently enough these days that the
  bundled versions are fine
- Rename nightly/create_tarball.sh -> nightly/make_snapshot_tarball
- Add "snapshot" and "snapshot_version" fields in VERSION; removed
  want_repo_rev and repo_rev fields
- Updated make_dist_tarball and make_snapshot_tarball to handle git
- Do not handle SVN or Mercurial repos any more
- Somewhat simplified hwloc_get_version.sh:
  - Never fill in a repo version number; always just report what is in
    VERSION already (and error out if there's not enough information
    in VERSION).  Only distscript.csh will insert a value into
    VERSION:snapshot_version (when needed).
  - Removed extra CLI options (e.g., get major/minor/etc. version
    number components)