Be more liberal in accepting Signed-off-by lines

Android kernel team sometimes needs to cherry-pick patches from the
mainline kernel, patches that have already been Signed-off-by the
author and Linus Torvalds.  In such cases requiring that the Android
developer add their own Signed-off-by line to an already existing
commit from mainline is rather useless, so we relax the requirement.

Requested-by: Brian Swetland <swetland@google.com>
Signed-off-by: Shawn O. Pearce <sop@google.com>