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.

Separate the concerns of the hash functions from the git_oid functions.
The git_oid structure will need to understand either SHA1 or SHA256; the
hash functions should only deal with the appropriate one of these.

Make some syntax change to follow coding style.

This change fixes a possible null pointer dereference if anything inside
`git_commit_graph_writer_add_index_file` fails before the packfile being
valid.

https://scan6.coverity.com/reports.htm#v52218/p10377/fileInstanceId=122935896&defectInstanceId=32525576&mergedDefectId=1461634

Also, add `git_commit_graph_writer_options_init`!

* Added the `PenaltyBreakAssignment: 1000` clang-format option to avoid
  breaking statements around the assignment operator.
* Avoided using the dot initializer syntax.
* Avoided casting allocations.
* Also avoided casting `void *`.

This change adds the git_commit_graph_writer_* functions to allow to
write and create `commit-graph` files from `.idx`/`.pack` files or
`git_revwalk`s.

Part of: #5757