Bug#831521: Custom repacking

[Osamu Aoki]

control: tags -1 - patch
control: severity -1 wishlist
control: retitle -1 implement capability to run custom mk-origtargz
control: severity 858319 wishlist
control: retitle 858319 implement capability to run custom mk-origtargz
control: merge 858319 831521

Hi,

Just because uupdate entry in watch file enables to run custom script,
abusing it is not good idea.

uscan deligates its key functions to:
 * repacking of upstream tarball/signature -- mk-origtargz
 * updating new sorce tree                 -- uupdate

So adding uscan a capability to run custom script in place of calling
mk-origtargz is the right approach.

That's exactly like repack.stub discussed in # 858319

I need to address these bugs without making overcomplicated uscan to do
more work.  Maybe if debian/mk-origtargz exists, it is used instead of
the default one.   That's simple.  This also enables if someone have
good script, I can merge into mk-origtargz.

Regards,

Osamu