We set an error if we get an error when reading, but we don't bother
setting an error message for write failing. This causes a cryptic error
to be shown to the user when the target filesystem is full.

Now that `git_path_direach` lets us specify an error message to report,
set an appropriate error message while linking.

This function deals with functions doing IO which means the amount of
errors that can happen is quit large. It does not help if it always
ovewrites the underlying error message with a less understandable
version of "something went wrong".

Instead, only use this generic message if there was no error set by the
callback.

Updated dlibgit link.

Bring Early EOF message in line with the other two

Add tests for and fix push negotiation notification

There was a copypasta error and the source and destination IDs were
reversed.