The git provider had some problems checking out SHA1s - it couldn't.
And what's worse, it lied about what it was doing saying that it did
checkout the SHA1.
There was also a bug where if you specified a different branch than you
were on, it was doing a `reset --hard` to the specified branch. For
example, if master was checked out, and you set "revision => stable", it
would `git reset --hard stable` on the master branch instead of just
checking out stable. Maybe the original author did this to get around
being unable to checkout when you had local changes, but the --force
flag to checkout will fix that.
With this change, you should now be able to specify a branch, tag, or
SHA1 in the revision attribute and have it work.
b931190b32