Save backup using non-destructive stash.

Update the logic to use my fork of git2-rs and libgit2, which expose
stash create and stash store operations so that using a stash to take
the backup does not change files on the file system.

This is friendlier to projects which have file watchers that react
to changes in the file system.

Use GNU tar on OSX runners.

Use GNU tar due to a strange issue with BSD tar on OSX runners.

For more information, see: actions/runner-images#2619

Update release.yml