config-item: declare undeclared path suboption

This should prevent issue like the previous commit.

mercurial/configitems.py

@@ -1851,7 +1851,7 b' coreconfigitem('
 )
 coreconfigitem(
     b'paths',
-    b'.*',
+    b'[^:]*',
     default=None,
     generic=True,
 )