extensions: obsolete and remove interhg extension
With the addition of the websub filter extension this extension is no longer
needed. We maintain a sort of backwards compatibility by reading the [interhg]
section and using it as we would use the [websub] section.
hide outer repo
$ hg init
$ echo '[Section]' >> $HGRCPATH
$ echo 'KeY = Case Sensitive' >> $HGRCPATH
$ echo 'key = lower case' >> $HGRCPATH
$ hg showconfig Section
Section.KeY=Case Sensitive
Section.key=lower case