gnu diff3: apply patch to committed src, rather than at build time

This appears to be a leftover from the time we used a less-capable VCS.

Reviewed by: delphij
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37368

(cherry picked from commit 3a13b5ac83a100ebb5a0da0579e418f19b3b630b)