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`.

`git_diff_format_email` is deprecated in favor of `git_email_create`.

buf: deprecate public git_buf writing functions

A `git_buf` is now a read-only structure as far as callers are
concerned.  This is a mechanism that we can return data to callers
using memory that is owned by the library and can be cleaned up by
callers (using `git_buf_dispose`).

A `git_buf` can no longer be allocated by callers or provided to the
library.

The signing callback should not be used; instead, callers should provide
a commit_create_cb, perform the signing and commit creation themselves.

Deprecate `git_filter_list_apply_to_data` as it takes user input as a
`git_buf`.  Users should use `git_filter_list_apply_to_buffer` instead.

`git_filter_list_stream_data` takes user input in a `git_buf`.
`git_buf` should only be used when libgit2 itself needs to allocate data
and returned to a user that they can free when they wish.  Replace it
with `git_filter_list_stream_buffer` that takes a data buffer and
length.

The function `git_treebuilder_write_with_buffer` is unnecessary; it
is used internally as part of treebuilder writing, but it has little
use to external callers.  For callers that repeatedly write a
treebuilder, we can supply them with a buffer in the treebuilder struct
instead of recreating it.  For ourselves, when we want a single buffer
in our write loop, we can use an internal function.

The information about the type of a revision spec is not information
about the parser.  Name it accordingly, so that `git_revparse_mode_t`
is now `git_revspec_t`.  Deprecate the old name.