Infrastructure:
* Properly honour NOCOMMITCHECK policy hook exit status.
Closes:#851800.
+ * Do not reject commits with no author/committer name (but still insist
+ on email address and date). Peter Green reports that eg
+ 71e128629ec786f3 in upstream xen.git is such a commit (and is accepted
+ by github). Closes:#851716.
Test suite:
* downstream-gitless: Test import of .dsc from unknown distro.