apply: Test for git_apply_to_tree failures when new files are added
Introduce an unit test to validate if git_apply_to_tree() fails when an
applied patch adds new files.
apply: move location to an argument, not the opts
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.
apply: introduce `git_apply_tree`
Introduce `git_apply_tree`, which will apply a `git_diff` to a given
`git_tree`, allowing an in-memory patch application for a repository.