largefiles: fix the directory structure when archiving a subrepo in a subrepo
Previously, a repo consisting of main/sub/subsub archived sub and subsub as
siblings under main.
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