We currently recommend using `git_buf_grow` in order to make a buffer
make an owned copy of the memory it points to. This is not behaviour we
should encourage, so remove this recommendation.

The function itself is not changed, as we need to remain compatible, but
it will be changed not to allow usage on borrowed buffers.

filter.h tells me that we check the first 8000 bytes.

The callback to supply data chunks could return a negative value
to stop creation of the blob, but we were neither using GIT_EUSER
nor propagating the return value.  This makes things use the new
behavior of returning the negative value back to the user.

This makes the git_buf struct that was used internally into an
externally available structure and eliminates the git_buffer.

As part of that, some of the special cases that arose with the
externally used git_buffer were blended into the git_buf, such as
being careful about git_buf objects that may have a NULL ptr and
allowing for bufs with a valid ptr and size but zero asize as a
way of referring to externally owned data.

This begins the process of exposing git_filter objects to the
public API.  This includes:

* new public type and API for `git_buffer` through which an
  allocated buffer can be passed to the user
* new API `git_blob_filtered_content`
* make the git_filter type and GIT_FILTER_TO_... constants public

This removes the GIT_INLINE versions of the simple git_object
accessors and standardizes them with a helper macro in src/object.h
to build the function bodies.