mail/nmh-devel: Fetch from SAVANNAH snapshot instead of my GH mirror

After discussion with Ken Hornstein, the nmh upstream maintainer,
git.savannah.gnu.org supports fetching snapshots using a git hash.
Therefore switch from using my nmh GH mirror to fetching snapshots
directly from git.savannah.gnu.org.

Dissued with: Ken Hornstein <kenh@cmf.nrl.navy.mil> (upstream maintainer)