diff --submodule: split into bite-sized pieces

Introduce two functions:

 - prepare_submodule_summary prepares the revision walker
   to list changes in a submodule.  That is, it:

   * finds merge bases between the commits pointed to this
     path from before ("left") and after ("right") the change;
   * checks whether this is a fast-forward or fast-backward;
   * prepares a revision walk to list commits in the symmetric
     difference between the commits at each endpoint.

   It returns nonzero on error.

 - print_submodule_summary runs the revision walk and saves
   the result to a strbuf in --left-right format.

The goal is just readability.  No functional change intended.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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