Bug#946002: git-buildpackage: import-orig into current branch or warn if current branch is not master

[Drew Parsons]

Package: git-buildpackage
Version: 0.9.28
Followup-For: Bug #946002
Control: severity -1 important

This bug has yet again corrupted a working local git repo, forcing a
reclone from salsa.  So raising severity to important.