migrate: support cmdline options for Mageia SVN selection

With this, we can select which Mageia version and/or repository
we will migrate packages from.
migrate: expand variables w/o brackets

Until now, we had supported the "%{variable}" syntax.
Now, we also support the "%variable" one
migrate: support named sections

Like the "%description sub-package" etc.
migrate: remove print statement
migrate: when replacing vars, repeat until all %'s are resolved
migrate: set '%name' variable from "Name:" section
migrate: allow steps to be skipped or be executed one-at-a-time

When things go wrong, manual intervention is required. We may need
to skip a step that we have manually performed, or just stop before
some step would go astray.
migrate: preliminary support for patches

Detect them at the RPM header, also keep the comments above a patch
as the commit message.

Then, have a special step[*] which does 'patch -pX -i file.patch'
and then commit the changes.

[*] since many projects ship with git-patches, we should ideally
detect their format and use 'git am' instead.