libgit2 has two distinct requirements that were previously solved by
`git_buf`. We require:
1. A general purpose string class that provides a number of utility APIs
for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
can take ownership of.
By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.
Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class. The name also
is an homage to Junio Hamano ("gitstr").
The public API remains `git_buf`, and has a much smaller footprint. It
is generally only used as an "out" param with strict requirements that
follow the documentation. (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)
Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.
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 moves the git_filter_list into the public API so that users
can create, apply, and dispose of filter lists. This allows more
granular application of filters to user data outside of libgit2
internals.
This also converts all the internal usage of filters to the public
APIs along with a few small tweaks to make it easier to use the
public git_buffer stuff alongside the internal git_buf.
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
