git-submodule summary: show commit summary

This patch does the hard work to show submodule commit summary.

For a modified submodule, a series of commits will be shown with
the following command:

    git log --pretty='format:%m %s' \
        --first-parent sha1_src...sha1_dst

where the sha1_src is from the given super project commit and the
sha1_dst is from the index or working tree (switched by --cached).

For a deleted, added, or typechanged (blob<->submodule) submodule,
only one single newest commit from the existing end (for example,
src end for submodule deleted or type changed from submodule to blob)
will be shown.

If the src/dst sha1 for a submodule is missing in the submodule
directory, a warning will be issued except in two cases where the
submodule directory is deleted (type 'D') or typechanged to blob
(one case of type 'T').

In the title line for a submodule, the src/dst sha1 and the number
of commits (--first-parent) between the two commits will be shown.

The following example demonstrates most cases.

    Example: commit summary for modified submodules sm1-sm5.
    --------------------------------------------
    $ git submodule summary
    * sm1 354cd45...3f751e5 (4):
      < one line message for C
      < one line message for B
      > one line message for D
      > one line message for E

    * sm2 5c8bfb5...000000 (3):
      < one line message for F

    * sm3 354cd45...3f751e5:
      Warn: sm3 doesn't contain commit 354cd45

    * sm4 354cd34(submodule)-> 235efa(blob) (1):
      < one line message for G

    * sm5 354cd34(blob)-> 235efa(submodule) (5):
      > one line message for H

    --------------------------------------------

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

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