Fix bug in --path-rename argument without colon

The --path-rename flag expected an argument with a colon
character (':') in it, which it assumed without checking. If the user
gave an argument with no colon in it, this backtrace would be shown:

  File "/usr/local/bin/git-filter-repo", line 1626, in __call__
    if values[0] and values[1] and not (
IndexError: list index out of range

Add a real error message in place of the backtrace.

Also check that there's exactly one colon; show an error message if
there's more than one, as that syntax has no interpretation that is
obviously the right one.

Signed-off-by: Lassi Kortela <lassi@lassi.io>

git-filter-repo

@@ -1622,7 +1622,10 @@ class FilteringOptions(object):
       if suffix.startswith('rename'):
         mod_type = 'rename'
         match_type = option_string[len('--path-rename-'):] or 'match'
-        values = values.split(b':', 1)
+        values = values.split(b':')
+        if len(values) != 2:
+          raise SystemExit(_("Error: --path-rename expects one colon in its"
+                             " argument: <old_name:new_name>."))
         if values[0] and values[1] and not (
            values[0].endswith(b'/') == values[1].endswith(b'/')):
           raise SystemExit(_("Error: With --path-rename, if OLD_NAME and "