Conflicts:
	src/refspec.c

This also includes droping `git_buf_lasterror` because it makes no sense
in the new system. Note that in most of the places were it has been
dropped, the code needs cleanup. I.e. GIT_ENOMEM is going away, so
instead it should return a generic `-1` and obviously not throw
anything.

Signed-off-by: schu <schu-github@schulog.org>

git_refspec_transform_r assumed that the reference name passed would
be only a branch or tag name. This is not the case, and we need to
take into consideration what's in the refspec's source to know how
much of the prefix to ignore.