rpm: support squashing commits in patch generation

rpm: support squashing commits in patch generation

Implements an option for git-buildpackage-rpm and gbp-pq to squash
commits (from upstream) up to certain tree-ish into one monolithic diff.
Useful e.g. if you wan't to auto-generate a stable update patch.

The new format of the cmdline option filename is commit-ish followed by
(optionally) a colon and the desired diff filename base. Suffix '.diff'
is added by GBP.

Magic word 'HEAD' translates to the end-commit when given as the
squash-point.  This allows one to configure gbp to always squash all
commits into one monolithic diff.

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