Updates: teaching git-add--interactive to accept a file param

Here are some incremental updates incorporating the feedback received on the
patch series that I sent earlier today. These apply on top of the previous
patches; when this series matures I'll rebase and squash this down to a
smaller number of commits, but for the timebeing I think it's clearer if I
post these modifications separately so they can be reviewed in themselves.

The modifications are:

      Suppress spurious linefeeds in git-add--interactive
      Teach git-add--interactive to accept multiple file params
      Teach builtin-add to pass multiple paths to git-add--interactive
      Update git-add documentation for multiple interactive paths

Next step will be to follow-up on Junio's suggestion that we should support
directory paths, not just file paths, and replicate the semantics of
non-interactive git-add.

 Documentation/git-add.txt |    4 ++--
 builtin-add.c             |   23 +++++++++++++----------
 commit.h                  |    2 +-
 git-add--interactive.perl |    5 ++---
 4 files changed, 18 insertions(+), 16 deletions(-)