Prior to this, it would fail if the 2nd SHA wasn't in the local repo.
Now it doesn't care what the local SHA for rev is, it only cares what is
checked out at HEAD.
Also deref tags on ls-remote
The short story: `ls-remote` for a tag gets us the SHA of the tag, but
`rev-parse HEAD` gets us the SHA of the commit to which that tag is
attached. Those are never equal, so we detect "update needed" every
loop.
Now we ask `ls-remote` for the rev and the dereferenced rev. If that
rev is a branch, the deref does nothing. If that rev is a tag it
produces both results. ls-remote does its own sort, so the deref (if
found) comes after the non-deref. This means that, in both cases, the
last line is the one we want.
3eb34e058c