The code surrounding zlib bundling did not take into consideration
that ZLIB_LIBRARY gets cached, and assumed that FIND(ZLIB) would
always set ZLIB_FOUND, which does not hold true, as this variable
signifies that we have found the package and had to look at the
system, as its location was not cached.

Only use the bundled sources if the external zlib is neither
newly-found nor cached.

Fix refdb iteration early termination bug

There was a problem found in the Rugged test suite where the
refdb_fs_backend__next function could exit too early in some
very specific hashing patterns for packed refs.  This ports
the Rugged test to libgit2 and then fixes the bug.

Fix broken build when MSVC SDL checks is enabled

Introduce a refs iterator

If the backend doesn't provide support for it, the matching is done in
refdb on top of a normal iterator.

Nobody should ever be using anything other than ALL at this level, so
remove the option altogether.

As part of this, git_reference_foreach_glob is now implemented in the
frontend using an iterator. Backends will later regain the ability of
doing the glob filtering in the backend.