`modified-only` may triggers twice some tests

[LecrisUT]

This is extracted from #4813 (comment)

The code path in question is

tmt/tmt/steps/discover/fmf.py

Lines 794 to 802 in 713f98c

	directories = [Path(name).parent for name in output.stdout.split('\n')]
	modified = {
	f"^/{re.escape(str(directory))}" for directory in directories if directory
	}
	if not modified:
	# Nothing was modified, do not select anything
	return []
	self.debug(f"Limit to modified test dirs: {modified}", level=3)
	self.data.test.extend(TestsWithAdjusts(name=name) for name in modified)

If we consider a plan structure like

$ tmt plans ls
/foo/py313
/foo/py314
$ tree .
.
└── foo
    ├── main.fmf
    ├── py313.fmf
    └── py314
        └── main.fmf

And we add 2 files so that the structure is now

$ tree .
.
└── foo
    ├── file.txt
    ├── main.fmf
    ├── py313.fmf
    └── py314
        ├── file.txt
        └── main.fmf

The directories above resolves to foo and foo/py314. This creates a problem when these are then used to resolve tree.tests(name) because it would run the tests for

        summary: 3 tests selected
            /foo/py313
            /foo/py314
            /foo/py314

(/foo/py314 appearing twice)

---

The best solution I have for this is to only pass it the change closest to the fmf root (only foo in this case). This needs some additional changes since we are currently implicitly only handling the fmf root is the same as the git root, but for this we need to relax this hidden assumption or better account for it

$ git log --format= --stat --name-only HEAD^..HEAD
fmf_tree/foo/file.txt
fmf_tree/foo/py314/file.txt

(the fmf_tree there is unexpected, we want it to have resolved to foo/file.txt and foo/py314/file.txt)