Problem:
Kernel patches for 4.12+ have been moved to a new location and are no longer compressed.
However, the current kernel-2.eclass is only aware of the old URI.
Steps to reproduce:

emerge -1v '>=sys-kernel/git-sources-4.12_rc1'

Outcome:
Kernels >= 4.12 cannot be installed due to incorrect patch URI.
Eg. Portage attempts to download a non-existing patch-4.12-rc3.xz from http://distfiles.gentoo.org/distfiles/ and https://www.kernel.org/pub/linux/kernel/v4.x/testing/

Attempts to update manifest fail for same reason

Possible Resolution:
Pull in Gentoo's upstream version
Upstream version: https://gitweb.gentoo.org/repo/gentoo.git/plain/eclass/kernel-2.eclass

Main change is in the kernel patch path at ~line 510. Other changes are removal of the parentheses in the function declaration on line 1454: kernel-2_src_unpack, and removal of trailing white-space throughout the eclass (which might as well be removed for cleaning purposes).

Tested and working on my system.