buildpackage-rpm: patch-export from development branch

buildpackage-rpm: patch-export from development branch

Adds git-buildpackage-rpm --patch-export support for the 'orphan
packaging' development model (where packaging files and development sources are kept in separate
branches).

New functionality:
1. If patch-export is enabled and gbp detects that the current branch
   has a development/patch-queue branch it exports the patches from there,
   instead of the tip of the packaging branch.
2. If gbp detects that the current (or exported) branch is a
   development/patch-queue branch it automatically enables patch-export
   and exports packaging files from the base branch (instead of the
   development/patch-queue branch.

Also, add a new '--git-patch-export-rev' command line option with which
the user can explicitly set the treeish from which patches are generated
(i.e. HEAD..<patch-export-rev>)

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