Fixed the filter order to match core Git, too.

This test demonstrates an interesting behavior of core Git (which
is totally reasonable and which libgit2 matches, although mostly
by coincidence).  If you use the ident filter and commit a file
with a garbage ident in it, like '$Id: this is just garbage$' and
then immediately do a 'git checkout-index' with the new file, Git
will not consider the file out of date and will not overwrite the
file with an updated $Id$.  Libgit2 has the same behavior.  If you
remove the file and then do a checkout-index, it will be replaced
with a filtered version that has injected the OID correctly.

Unfortunately git-core uses the term "unborn branch" and "orphan
branch" interchangeably. However, "orphan" is only really there for
the checkout command, which has the `--orphan` option so it doesn't
actually create the branch.

Branches never have parents, so the distinction of a branch with no
parents is odd to begin with. Crucially, the error messages deal with
unborn branches, so let's use that.