add: fix subrepo recursion for explicit path handling When support for handling explicit paths in subrepos was added to the add command (9e99d2bbb1b1), subrepo recursion wasn't taken into account. This change adds an explicitonly argument to cmdutil.add to allow controlling which levels of recursion should include only explicit paths versus all matched paths.

  $ echo '[Section]' >> $HGRCPATH
  $ echo 'KeY = Case Sensitive' >> $HGRCPATH
  $ echo 'key = lower case' >> $HGRCPATH

  $ hg showconfig Section
  Section.KeY=Case Sensitive
  Section.key=lower case