pq-rpm: create diff up to the last merge commit

Before, merge commits broke patch generation, or, (most probably) caused
the generated patches to be unapplicable (similar to how
git-format-patch behaves).

Now, changes up to the last merge commit are squashed into one diff if
merge commits are found in the revision list from which patches are to
be generated. Individual patches (one per commit) are generated from the
last merge commit up to the exported revision.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>