filter-repo: permit trailing slash for --[to-]subdirectory-filter argument

There was code to allow the argument of --to-subdirectory-filter and
--subdirectory-filter to have a trailing slash, but it was broken due to
a bug in python3's bytestring design: b'somestring/'[-1] != b'/',
despite that being the obvious expectation.  One either has to compare
b'somestring/'[-1:] to b'/' or else compare b'somestring/'[-1] to
b'/'[0].  So lame.  Note that this is essentially a follow-up to commit
385b0586ca ("filter-repo (python3): bytestr splicing and iterating is
different", 2019-04-27).

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

git-filter-repo

@@ -1632,7 +1632,7 @@ class FilteringOptions(object):
     def __call__(self, parser, namespace, values, option_string=None):
       af = FilteringOptions.AppendFilter(dest='path_changes',
                                          option_strings=None)
-      dirname = values if values[-1] == b'/' else values+b'/'
+      dirname = values if values[-1:] == b'/' else values+b'/'
       if option_string == '--subdirectory-filter':
         af(parser, namespace, dirname,     '--path-match')
         af(parser, namespace, dirname+b':', '--path-rename')

t/t9390-filter-repo.sh

@@ -246,12 +246,30 @@ test_expect_success '--subdirectory-filter' '
 	)
 '
 
+test_expect_success '--subdirectory-filter with trailing slash' '
+	(
+		git clone file://"$(pwd)"/metasyntactic subdir_filter_2 &&
+		cd subdir_filter_2 &&
+		git filter-repo \
+			--subdirectory-filter words/ &&
+		git cat-file --batch-check --batch-all-objects >all-objs &&
+		test_line_count = 10 all-objs &&
+		git log --format=%n --name-only | sort | uniq >filenames &&
+		test_line_count = 6 filenames &&
+		grep ^important$ filenames &&
+		test_must_fail git cat-file -t v1.0 &&
+		test_must_fail git cat-file -t v1.1 &&
+		test $(git cat-file -t v2.0) = commit &&
+		test $(git cat-file -t v3.0) = tag
+	)
+'
+
 test_expect_success '--to-subdirectory-filter' '
 	(
 		git clone file://"$(pwd)"/metasyntactic to_subdir_filter &&
 		cd to_subdir_filter &&
 		git filter-repo \
-			--to-subdirectory-filter mysubdir &&
+			--to-subdirectory-filter mysubdir/ &&
 		git cat-file --batch-check --batch-all-objects >all-objs &&
 		test_line_count = 22 all-objs &&
 		git log --format=%n --name-only | sort | uniq >filenames &&