This makes the API for commiting or discarding changes the same as for
references.

This lock/unlock pair allows for the cller to lock a configuration file
to avoid concurrent operations.

It also allows for a transactional approach to updating a configuration
file. If multiple updates must be made atomically, they can be done
while the config is locked.

When a configuration file is locked, any updates made to it will be done
to the in-memory copy of the file. This allows for multiple updates to
happen while we hold the lock, preventing races during complex
config-file manipulation.

Instead of writing into the filebuf directly, make the functions to
write the modified config file write into a buffer which can then be
dumped into the lockfile for committing.

This allows us to re-use the same code for modifying a locked
configuration, as we can simply skip the last step of dumping the data
to disk.

Forcing libssh2 lib location in CMake

OS X may have libssh2 in diff locations, so CHECK_LIBRARY_EXISTS may check the wrong lib; forcing it to use a found directory.

Fix duplicate basenames to support older VS on master

With Visual Studio versions 2008 and older they ignore the full path to files and only check
the basename of the file to find a collision. Additionally, having duplicate basenames can break
other build tools like GYP.

This fixes https://github.com/libgit2/libgit2/issues/3356

Error handling: use buffers, improved OOM handling

When an error state is an OOM, make sure that we treat is specially
and do not try to free it.