Split rewrites, status doesn't return rewrites

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".
Some documentation improvements
Merge pull request #1805 from libgit2/threading-packed-load

Thread safety for the refdb_fs
Add stddef include for sortedcache

All use of sortedcache will need this header, so put it in the
definition of the sortedcache API.
Fix incorrect precedence within git_repository_is_empty()

Reverts part of 9146f1e57ec4f2b6fa293c78d54f1383464ff5be.
Fix typo _delete -> _free
repository: Make the is_empty check more explicit
Treat detached HEAD as non-empty repo

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.
odb: Honor the non refreshing capability of a backend
odb: Code beautification
Update documentation of git_oid_streq to remove outdated error code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. Namely: https://github.com/carlosmn/geef
Merge pull request #1821 from n1rvana/development

Update readme to point to the currently maintained Erlang bindings.
Point to the right Go bindings
Merge pull request #1819 from linquize/git_oid_shorten_add

oid: git_oid_shorten_add() sets GITERR_INVALID when OID set is full
Merge pull request #1820 from linquize/git_oid_streq

Update documentation of git_oid_streq to remove outdated error code
Create ANDROID build option

CMake seems not to support Android as a target and this option
lets us test this in CMakeLists.txt.
Use git__insertsort_r on Android too.
Define S_IREAD i S_IWRITE for Android.