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.

oidarray: introduce `git_oidarray_dispose`

`git_strarray` is a public-facing type.  Change
`git_buf_text_common_prefix` to not use it, and just take an array of
strings instead.

Since users are disposing the _contents_ of the oidarray, not freeing
the oidarray itself, the proper cleanup function is
`git_oidarray_dispose`.  Deprecate `git_oidarray_free`.

The `repo` argument is now unnecessary.  Remove it.

When `git_repository_hashfile` is handed an absolute path, it determines
whether the path is within the repository's working directory or not.
This is necessary when there is no `as_path` specified.

If the path is within the working directory, then the given path should
be used for attribute lookups (it is the effective `as_path`).  If it is
not within the working directory, then it is _not_ eligible.

Importantly, now we will _never_ pass an absolute path down to attribute
lookup functions.