[PATCH 0 of 3] add: support adding explicit files in subrepos

[David M. Carr]

In the past couple months, there's been a lot of discussion about subrepos,
difficulties in using them today, and the path forward.  Several times, Matt
Mackall has requested that someone assemble a patch to have "hg add"
transparently handle adding explicit files in subrepos.  Thus, here is an
attempt at such a patch.

This is my first attempt at a patch for Mercurial (or any software written in
Python, for that matter).  I welcome any and all comments, critiques, or
suggestions for improvement.

This is somewhat related to Issue687, which requests that instead of skipping
subrepo adds silently, Mercurial should alert the user to the sitation.  If we
go forward with the subrepo add actually working, this ticket can likely be
marked as resolved.