filter-repo: fix --use-mailmap

--use-mailmap was defined as `--mailmap .mailmap` except that it would
set args.mailmap to ".mailmap" rather than b".mailmap" (in other words,
it accidentally set it to a string rather than a bytestring).  Since
the --mailmap parameter is always passed as a bytestring, we ran into
errors with calling unknown functions due to the type mismatch.

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

git-filter-repo

@@ -1862,7 +1862,7 @@ EXAMPLES
                "part of git history, historical versions of the file will "
                "be ignored; only the current contents are consulted."))
     people.add_argument('--use-mailmap', dest='mailmap',
-        action='store_const', const='.mailmap',
+        action='store_const', const=b'.mailmap',
         help=_("Same as: '--mailmap .mailmap' "))
 
     parents = parser.add_argument_group(title=_("Parent rewriting"))