Test root gitignore behavior on multiple sources

The test creates a fake context and collects the files from two
sources. Both sources are directories in the root path, where a
gitignore file ignores a filename that is present in both subdirs.

Before the fix introduced in the previous commit, this test was
expected to fail: a file that should be ignores was still visible for
Black. Now, the test is passed.

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>

tests/data/gitignore_used_on_multiple_sources/.gitignore

@@ -0,0 +1 @@
+a.py

tests/test_black.py

@@ -1956,6 +1956,17 @@ def test_gitignore_used_as_default(self) -> None:
         ctx.obj["root"] = base
         assert_collected_sources(src, expected, ctx=ctx, extend_exclude=r"/exclude/")
 
+    def test_gitignore_used_on_multiple_sources(self) -> None:
+        root = Path(DATA_DIR / "gitignore_used_on_multiple_sources")
+        expected = [
+            root / "dir1" / "b.py",
+            root / "dir2" / "b.py",
+        ]
+        ctx = FakeContext()
+        ctx.obj["root"] = root
+        src = [root / "dir1", root / "dir2"]
+        assert_collected_sources(src, expected, ctx=ctx)
+
     @patch("black.find_project_root", lambda *args: (THIS_DIR.resolve(), None))
     def test_exclude_for_issue_1572(self) -> None:
         # Exclude shouldn't touch files that were explicitly given to Black through the