Threading can now be disabled with `USE_THREADS=OFF` instead of
`THREADSAFE=OFF` to better support the other cmake semantics.

Nanosecond support is the default _if_ we can detect it.  This should be
our default always - like threads - and people can opt out explicitly.

in #6083 the test runner was renamed to libgit2_tests,
but not all references to the old name were updated.
this change changes all of them to use the new name.

Nobody knows what CLAR is.  The test building option should be
`BUILD_TESTS`.

We have been inconsistent about the way that we handle `git_buf`s
provided by users.  _Usually_ we require that it has been properly
initialized with `GIT_BUF_INIT`, but _sometimes_ we simply overwrite
the data in it regardless.  And even more rarely, we will grow a
user-provided buffer and concatenate data onto it (see
`git_diff_format_email`).

Document the path forward for `git_buf`, which is that we always
require that the buffer is intitialized with `GIT_BUF_INIT`.

`git_diff_format_email` will be kept backward compatible but users
are encouraged to switch to the new `git_email` APIs.