The pair of `git_blob_create_frombuffer()` and
`git_blob_create_frombuffer_commit()` is meant to replace
`git_blob_create_fromchunks()` by providing a way for a user to write a
new blob when they want filtering or they do not know the size.

This approach allows the caller to retain control over when to add data
to this buffer and a more natural fit into higher-level language's own
stream abstractions instead of having to handle IO wait in the callback.

The in-memory buffer size of 2MB is chosen somewhat arbitrarily to be a
round multiple of usual page sizes and a value where most blobs seem
likely to be either going to be way below or way over that size. It's
also a round number of pages.

This implementation re-uses the helper we have from `_fromchunks()` so
we end up writing everything to disk, but hopefully more efficiently
than with a default filebuf. A later optimisation can be to avoid
writing the in-memory contents to disk, with some extra complexity.