Some windows tests were failing

It seems that libgit2 is correctly applying the umask when
initializing a repository from a template and when creating new
directories during checkout, but the test suite is not accounting
for possible variations due to the umask.  This updates that so
that the test suite will work regardless of the umask.

Of backends and refreshers...

Previously, `git_object_read()`, `git_object_read_prefix()` and
`git_object_exists()` were implementing an auto refresh logic. When the
expected object couldn't be found in any backend, a call to
`git_odb_refresh()` was triggered and the lookup was once again performed
against all backends.

This commit removes this auto-refresh logic from the odb layer and pushes
it down into the pack-backend (as it's the only one currently exposing
a `refresh()` endpoint).

Split examples CMakeLists.txt

Also, this converts the examples/CMakeLists.txt from explicitly
listing to just globbing for all the individual C files.

Fix resolving relative windows network paths

Minor changes for rewrites

path: properly resolve relative paths

Fix incorrect precedence within git_repository_is_empty()

Small changes enabling compiling libgit2 for Android.

Small cleanup in examples.

CMake seems not to support Android as a target and this option
lets us test this in CMakeLists.txt.

Update documentation of git_oid_streq to remove outdated error code

oid: git_oid_shorten_add() sets GITERR_INVALID when OID set is full

Update readme to point to the currently maintained Erlang bindings. 

Update readme to point to the currently maintained Erlang bindings. Namely: https://github.com/carlosmn/geef

This simplifies the git_repository_is_empty a bit so that a
detached HEAD is just taken to mean the repo is not empty, since
a newly initialized repo will not have a detached HEAD.