Signed-off-by: Sven Strickroth <email@cs-ware.de>

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.

putting `0.` at the start of the line turns it into a numbered list.

We currently recommend using `git_buf_grow` in order to make a buffer
make an owned copy of the memory it points to. This is not behaviour we
should encourage, so remove this recommendation.

The function itself is not changed, as we need to remain compatible, but
it will be changed not to allow usage on borrowed buffers.

filter.h tells me that we check the first 8000 bytes.