Re: [PATCH 1/3] t4117: check for files using `test_path_is_file`

> We `cat` files, but don't inspect or grab the contents in any way. These
> `cat` calls look like remnants from a debug session, so it's tempting to
> get rid of them. But they do actually verify that the files exist, which
> might not necessarily be the case for some failure modes of `git apply
> --reject`. Let's not lose that.
> 
> Convert the `cat` calls to use `test_path_is_file` instead. This is of
> course still a minor change since we no longer verify that the files can
> be opened for reading, but that is not something we usually worry about.