Make snapshots remember what git commit they are

Make snapshots remember what git commit they are

utils/snapshot now includes a file gitcommit in the archive, which
makeversion reads if it fails to detect a git repository.  It also
removes .gitignore and utils/, which are useless in a snapshot.

makeversion now only finds a git repository when .git exists in
the source directory, but won't find repositories at higher levels
anymore.  This is so that snapshots can be extracted and tested in
a subdirectory of the source, without having makeversion find the
git repository, but using the snapshot's gitcommit file instead.