rpm: add --orig-prefix option

rpm: add --orig-prefix option

Adds a new option --orig-prefix that affects the prefix of the
generated/imported orig tarballs.

For git-buildpackage-rpm, it defines the prefix of tarballs generated by
the tool, 'auto' (default) makes gbp to guess the prefix, as before.
NOTE: this doesn't affect the tarballs checked out from pristine-tar
(i.e. if --pristine-tar is defined) or tarballs that are already present
(see --git-tarball-dir option).

For git-import-orig, this new option affects the prefix of tarballs
imported into pristine-tar branch. If set to 'auto' (default) gbp
doesn't mangle the prefix.

Other tools (e.g. git-import-srpm) are not affected.

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