solvergit: use --unidiff-zero with git-apply(1)

solvergit: use --unidiff-zero with git-apply(1)

I sometimes post context-free documentation patches generated
with "-U0" to reduce size and bandwidth overhead when replacing
URLs or updating copyright notices.  git-apply(1) needs the
--unidiff-zero switch to work properly with context-free
patches.

Given our search looks for blob OIDs, and we're never going
to be running the code we regenerate, "--unidiff-zero" ought
to be safe.