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.

The `git_buf_text` namespace is unnecessary and strange.  Remove it,
just keep the functions prefixed with `git_buf`.

There was no test ensuring that we validate `.git` paths.  We do, but
let's add a test to make sure that we never regress this.

This adds an option which will check if a diff is applicable without
actually applying it; equivalent to git apply --check.

apply: Fix a patch corruption related to EOFNL handling

Introduce an unit test to validate that git_apply__patch() properly
handles EOFNL changes in case of patches with several hunks.

Introduce an unit test to validate if git_apply_to_tree() fails when an
applied patch adds new files.

Previously, we would fail to correctly truncate the source buffer
if the source has more than one line and ends with a non-newline
character. In the following call, we thus truncate the source
string in the middle of the second line. Without the bug fixed,
we would successfully apply the patch to the source and return
success. With the overflow being fixed, we should return an
error now.

Use the new-style index names throughout our own codebase.