Improve collection of information for format-patch --cover-letter

Improve collection of information for format-patch --cover-letter

Use the "boundary" feature to find the origin (or find that there are
multiple origins), and use the actual list of commits to pass to
shortlog.

This makes all cover letter include shortlogs, and all cover letters
for series with a single boundary commit include diffstats (if there
are multiple boundary commits it's unclear what would be meaningful as
a diffstat). Note that the single boundary test is empirical, not
theoretical; even a -2 limiting condition will give a diffstat if there's
only one boundary commit in this particular case.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>