Two tickets related to sdist (#12487 and #12488) need review

[Jeroen Demeyer]

Ticket #12487 deals with sage-dist, ticket #12488 deals with
sage-make_devel_packages. They can be reviewed independently.

Together they fix the following issues:

* race condition when doing sage --sdist VERSION simultaneously from 2
different source trees (by the same or a different user).

* much better error checking when something goes wrong in sage-sdist.

* doing sage --sdist VERSION for a VERSION which already exists in
.hgtags will now work (it used to fail because "hg commit" didn't have
anything to commit after "hg tag" and would return an exit status of 1).

So, please review
http://trac.sagemath.org/sage_trac/ticket/12487
http://trac.sagemath.org/sage_trac/ticket/12488

Thanks,
Jeroen.

[Jeroen Demeyer]

* bump *

Please review, these are important for release management.