repoman: Update header check for Gentoo repo policy.

By decision of the Gentoo Council in its 2019-01-13 meeting, ebuilds
in the Gentoo repository MUST use the simplified form of the copyright
attribution according to GLEP 76, i.e.: "Copyright YEARS Gentoo Authors".

Update the header check accordingly, mostly by reverting to the
simpler single line check that was in place before commit c4096aff48.

Bug: https://bugs.gentoo.org/666330
Signed-off-by: Ulrich Müller <ulm@kph.uni-mainz.de>
Signed-off-by: Zac Medico <zmedico@gentoo.org>
2 files changed