t9390: add a testcase for --path-rename with no colon

Commit 28b479b7 (Fix bug in --path-rename argument without colon,
2021-03-12) added a new conditional error message, with no corresponding
testcase to ensure the line was covered.  I forgot to check the coverage
before merging the change.  Add a relevant test now.

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

t/t9390-filter-repo.sh

@@ -1262,6 +1262,9 @@ test_expect_success 'other startup error cases and requests for help' '
 		test_must_fail git filter-repo --path-rename /foo:bar 2>err &&
 		test_i18ngrep "Pathnames cannot begin with a ./" err &&
 
+		test_must_fail git filter-repo --path-rename foo 2>err &&
+		test_i18ngrep "Error: --path-rename expects one colon in its argument" err &&
+
 		test_must_fail git filter-repo --subdirectory-filter /foo 2>err &&
 		test_i18ngrep "Pathnames cannot begin with a ./" err &&