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.

We should always verify error codes returned by function calls in our
test suite to not accidentally miss any weird results. Coverity reported
missing checks in several locations, which this commit fixes.

This also adds tests for this function.

This also adds tests for this function.

This also adds tests for this function.

This adds a new function that will allow creation of notes without
necessarily updating a particular ref, the notes tree is obtained
from the git_commit object parameter, a new commit object pointing
to the current tip of the notes tree is optionally returned
via the 'note_commit_out' parameter,
optionally the blob id for the note is returned through
the 'note_blob_out' object.

The caller has otherwise no way to know how long the string will be
allocated or ability to free it.

This fixes #2944.