t4117: check for files using `test_path_is_file`

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.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>