show_log: factor out interdiff/range-diff generation

show_log: factor out interdiff/range-diff generation

The integration of "git range-diff" with "git format-patch" for a
single patch (i.e., not generating "range-diff" into the cover
letter) hooks into log-tree.c:show_log(), which is responsible for
writing the log message out and other stuff.  Essentially,
everything you see before the diffstat and the patch is generated
there.

Split out the code that spits out the interdiff/range-diff into a
separate helper function show_diff_of_diff().  Hopefully this will
make it easier to move things around in the output stream in the
future patches.

This is supposed to be a no-op refactoring.

Signed-off-by: Junio C Hamano <gitster@pobox.com>