Fix issue with test suite and empty dirs on git

Git doesn't save empty dirs, so put a placeholder there that is
ignored when the test data is copied to its final location during
testing.

tests/fsck-data/test1/data/git-empty-dir-placeholder

@@ -0,0 +1 @@
+hi

tests/fsck-data/test1a/data/git-empty-dir-placeholder

@@ -0,0 +1 @@
+hi

tests/fsck-data/test1c/data/git-empty-dir-placeholder

@@ -0,0 +1 @@
+hi

tests/fsck-data/test2h/data/git-empty-dir-placeholder

@@ -0,0 +1 @@
+hi

tests/fsck-data/test2p/data/a/b/0001/git-empty-dir-placeholder

@@ -0,0 +1 @@
+hi

tests/test_fsck.py

@@ -18,7 +18,9 @@ class TestFsck(object):
         """
         if db is not None:
             recursive_unlink("tests/fsck-testdb")
-            shutil.copytree(f"tests/fsck-data/{db}", "tests/fsck-testdb")
+            shutil.copytree(f"tests/fsck-data/{db}", "tests/fsck-testdb",
+                            ignore=shutil.ignore_patterns(
+                                "git-empty-dir-placeholder"))
         class stdio_wrapper:
             def __init__(self, stdin, stdout, stderr):
                 self.io = (stdin, stdout, stderr)