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 adds the ident filter (that knows how to replace $Id$) and
tweaks the filter APIs and code so that git_filter_source objects
actually have the updated OID of the object being filtered when
it is a known value.
Extend the git2/sys/filter API with functions to look up a filter
and add it manually to a filter list. This requires some trickery
because the regular attribute lookups and checks are bypassed when
this happens, but in the right hands, it will allow a user to have
granular control over applying filters.
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
