fix could-not-access error

[jesseduffield]

• PR Description

fixes #1545

by passing the /dev/null arg to a git diff on a directory, we get a 'could not access' error. So we're not going to do that if we're dealing with a directory.

It would be cool however if we could show the untracked file as part of the diff, but I'm not sure how to do that. If we find a way, we should update the integration test that this PR introduces

This PR also fixes broken submodule tests in CI (See https://vielmetti.typepad.com/logbook/2022/10/git-security-fixes-lead-to-fatal-transport-file-not-allowed-error-in-ci-systems-cve-2022-39253.html)

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[aragon999]

Thank you for the fix :-)

Not sure if it helps, but using something like git status --untracked-files=all -s one could get a list of the files which are not tracked. Not sure how that fits into the current code though.

[jesseduffield]

@aragon999 it's not obvious how to use that to produce a single command that we can render the diff with. At any rate this PR solves the original problem.