Decode base64-encoded text into a git_buf

Allows for inserting the same character n amount of times

There are a few places where we need to join three strings to
assemble a path.  This adds a simple join3 function to avoid the
comparatively expensive join_n (which calls strlen on each string
twice).

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 creates include/sys/filter.h with a basic definition of a
git_filter and then converts the internal code to use it.  There
are related internal objects (git_filter_list) that we will want
to publish at some point, but this is a first step.