GH Action error

[MatthijsBurgh]

Describe the bug

argument PATH: Error: Path(s) '.' do not exist in 09237df1dd54d667598bf82b432a74f621f3d052

To Reproduce
Just running GH actions with the following options: ``
Which result in the following environment variables:
env:
INPUT_OPTIONS: --check --diff -l 120
INPUT_SRC: .
INPUT_VERSION: 1.5.0
INPUT_REVISION:
INPUT_LINT:
INPUT_COMMIT_RANGE: b2e1bfe99aa6c9e149ba6453a77770547bab4180...09237df1dd54d667598bf82b432a74f621f3d052
pythonioencoding: utf-8

Expected behavior
No error 😉

Screenshots
https://github.com/MatthijsBurgh/epydoc/runs/6154261435?check_suite_focus=true#step:3:90

Environment (please complete the following information):

• OS: Ubuntu 20.04.4
• Python version 3.8.10
• Git version 2.35.1
• Darker version 1.5.0
• Black version 22.3.0

Additional context
I think the problem is the INPUT_SRC path. Only indication for that is the matching dot.
update: I have no idea why it says it can't find the dot in the latest commit hash.
update2: Error is comming from

darker/src/darker/__main__.py

Lines 415 to 422 in ad6cde1

	missing = get_missing_at_revision(paths, revrange.rev2, root)
	if missing:
	missing_reprs = " ".join(repr(str(path)) for path in missing)
	rev2_repr = "the working tree" if revrange.rev2 == WORKTREE else revrange.rev2
	raise ArgumentError(
	Action(["PATH"], "path"),
	f"Error: Path(s) {missing_reprs} do not exist in {rev2_repr}",
	)

[akaihola]

I think we need to handle the repository root specially (i.e. always True):

>>> from pathlib import Path
>>> from darker.git import _git_exists_in_revision
>>> _git_exists_in_revision(Path("."), "HEAD", Path.cwd())
False
>>> _git_exists_in_revision(Path("src"), "HEAD", Path.cwd())
True