Convert pqueue to just be a git_vector

This updates the git_pqueue to simply be a set of specialized
init/insert/pop functions on a git_vector.

To preserve the pqueue feature of having a fixed size heap, I
converted the "sorted" field in git_vectors to a more general
"flags" field so that pqueue could mix in it's own flag.  This
had a bunch of ramifications because a number of places were
directly looking at the vector "sorted" field - I added a couple
new git_vector helpers (is_sorted, set_sorted) so the specific
representation of this information could be abstracted.
Fix typo setting sorted flag when reloading index

This fixes a typo I made for setting the sorted flag on the index
after a reload.  That typo didn't actually cause any test failures
so I'm also adding a test that explicitly checks that the index is
correctly sorted after a reload when ignoring case and when not.