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Update documentation of git_oid_streq to remove outdated error code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.

Reverts part of 9146f1e5.

All use of sortedcache will need this header, so put it in the
definition of the sortedcache API.

Thread safety for the refdb_fs

Ensure that we apply splits to rewrites, even if we're not
interested in examining it closely for rename/copy detection.

In keeping with core git, status should not display rewrites,
it should simply show files as "modified".

Even more lenient remote parsing