Calculation of where the include file lives was broken by 23acdd3f14
since it resulted in looking for the first include in .repo, rather
than .repo/manifests.
While people can work around it via setting their includes to
manifests/<include-target>, that breaks down since each layer of
includes would then have to be relative.
As such, restore the behaviour back to 2644874d; manifests includes
are calculated relative to the manifest root (ie, .repo/manifests);
local manifests includes are calculated relative to .repo/ .
Change-Id: I74c19ba614c41d2f08cd3e9fd094f3c510e3bfd1
475a47d531