configitems: use raw strings for hidden-{command,topic} items

These strings are regular expressions. The "\." needs to be
string escaped. We use raw strings to avoid doing that and the
SyntaxWarning we'd receive otherwise on Python 3.8.

Differential Revision: https://phab.mercurial-scm.org/D5822

mercurial/configitems.py

@@ -720,11 +720,11 b" coreconfigitem('fsmonitor', 'warn_when_u"
 coreconfigitem('fsmonitor', 'warn_update_file_count',
     default=50000,
 )
-coreconfigitem('help', 'hidden-command\..*',
+coreconfigitem('help', br'hidden-command\..*',
     default=False,
     generic=True,
 )
-coreconfigitem('help', 'hidden-topic\..*',
+coreconfigitem('help', br'hidden-topic\..*',
     default=False,
     generic=True,
 )