Be more careful with user-supplied buffers

Configuration snapshotting

This adds in missing calls to `git_buf_sanitize` and fixes a
number of places where `git_buf` APIs could inadvertently write
NUL terminator bytes into invalid buffers.  This also changes the
behavior of `git_buf_sanitize` to NUL terminate a buffer if it can
and of `git_buf_shorten` to do nothing if it can.

Adds tests of filtering code with zeroed (i.e. unsanitized) buffer
which was previously triggering a segfault.

Accessing the repository's config and immediately taking a snapshot of
it is a common operation, so let's provide a convenience function for
it.

Make sure we set the output parameter to a value.

This way we can assume we have a consistent view of the config situation
when we're looking up remote, branch, pack-objects, etc.

A few fixes have accumulated in this area which have made the freeing of
data a bit muddy. Make sure to free the data only when needed and once.

When we are going to write a delta to the packfile, we need to free the
data, otherwise leave it. The current version of the code mixes up the
checks for po->data and po->delta_data.

There were some confusing issues mixing up the number of bytes
written to the zstream output buffer with the number of bytes
consumed from the zstream input.  This reorganizes the zstream
API and makes it easier to deflate an arbitrarily large input
while still using a fixed size output.