git-submodule summary: code framework

git-submodule summary: code framework

These patches teach git-submodule a new subcommand 'summary' to show
commit summary of checked out submodules between a given super project
commit (defaults to HEAD) and working tree (or index, when --cached is
given).

This patch just introduces the framework to find submodules which have
summary to show. A submodule will have summary if it falls into these
cases:

  - type 'M': modified and checked out    (1)
  - type 'A': added and checked out       (2)
  - type 'D': deleted
  - type 'T': typechanged (blob <-> submodule)

Notes:

  1. There may be modified but not checked out cases. In the case of a
     merge conflict, even if the submodule is not checked out, there may
	 be still a diff between index and HEAD on the submodule entry
	 (i.e. modified). The summary will not be show for such a submodule.
  2. A similar explanation applies to the added but not checked out case.

Signed-off-by: Ping Yin <pkufranky@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>