contrib: check shasums for git snapshots tarballs

These tarballs (probably produces by git archive --format=tar) are
'fairly' stable. This means that git tries to not change the output of
this format, but changes happen occasionally due to bugfixes.

It's still a good idea to freeze the hash because we really want to know
when the contents of such a tarball might have changed.
A lot of other projects depend on these also (i.e. homebrew, buildroot,
etc.)

All hashes were locally computed.

Change-Id: Ib0b3fccab70236bd2eb7bbb236e1306af7cc5f36