Provide a public git_index_iterator API that is backed by an index
snapshot.  This allows consumers to provide a stable iteration even
while manipulating the index during iteration.

Introduce a callback to patch application that allows consumers to
cancel hunk application.

Introduce a callback to the application options that allow callers to
add a per-delta callback.  The callback can return an error code to stop
patch application, or can return a value to skip the application of a
particular delta.

Move the location option to an argument, out of the options structure.
This allows the options structure to be re-used for functions that don't
need to know the location, since it's implicit in their functionality.
For example, `git_apply_tree` should not take a location, but is
expected to take all the other options.

Return `GIT_EAPPLYFAIL` on patch application failure so that users can
determine that patch application failed due to a malformed/conflicting
patch by looking at the error code.

Introduce `git_apply`, which will take a `git_diff` and apply it to the
working directory (akin to `git apply`), the index (akin to `git apply
--cached`), or both (akin to `git apply --index`).

Introduce `git_apply_tree`, which will apply a `git_diff` to a given
`git_tree`, allowing an in-memory patch application for a repository.

Provide a utility to reset custom allocators back to their default.
This is particularly useful for testing.

These checks are preformed by libgit2 on checkout, but they're also useful for
performing checks in applications which do not involve checkout.

Expose them under `sys/` as it's still fairly in the weeds even for this
library.

The underlying code uses GIT_CONFIG_FILENAME_GLOBAL, which is .gitconfig.