filter-repo: warn users who try to use invalid path components

It's hard to be exhaustive, but if users try something like:
   --path-rename foo/bar/baz:.
or
   --path ../other-dir
then bad things happen.  In the first case, filter-repo will try to
ask fast-import to create a directory named '.' and move everything
from foo/bar/baz/ into it but of course '.' is a reserved directory
name so we can't create it.  In the second case, they are probably
running from a subdirectory, but filter-repo doesn't work from a
subdirectory.  I hard-coded the assumption that everything was in the
toplevel directory and all paths were relative from there pretty
early on.  So, if the user tries to use any of these components
anywhere, just throw an early error.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -1602,6 +1602,7 @@ class GitUtils(object):
 class FilteringOptions(object):
   class AppendFilter(argparse.Action):
     def __call__(self, parser, namespace, values, option_string=None):
+      user_path = values
       suffix = option_string[len('--path-'):] or 'match'
       if suffix.startswith('rename'):
         mod_type = 'rename'
@@ -1612,9 +1613,15 @@ class FilteringOptions(object):
           raise SystemExit(_("Error: With --path-rename, if OLD_NAME and "
                              "NEW_NAME are both non-empty and either ends "
                              "with a slash then both must."))
+        components = values[0].split(b'/') + values[1].split(b'/')
       else:
         mod_type = 'filter'
         match_type = suffix
+        components = values.split(b'/')
+      for illegal_path in [b'.', b'..', b'.git']:
+        if illegal_path in components:
+          raise SystemExit(_("Error: Invalid path component '%s' found in '%s'")
+                           % (decode(illegal_path), decode(user_path)))
       if match_type == 'regex':
         values = re.compile(values)
       items = getattr(namespace, self.dest, []) or []

t/t9390-filter-repo.sh

@@ -1028,7 +1028,10 @@ test_expect_success 'other startup error cases and requests for help' '
 		test_i18ngrep "renaming globs makes no sense" err &&
 
 		test_must_fail git filter-repo --strip-blobs-bigger-than 3GiB 2>err &&
-		test_i18ngrep "could not parse.*3GiB" err
+		test_i18ngrep "could not parse.*3GiB" err &&
+
+		test_must_fail git filter-repo --path-rename foo/bar:. 2>err &&
+		test_i18ngrep "Invalid path component .\.. found in .foo/bar:\." err
 
 	)
 '