grep: load funcname patterns for -W

grep: load funcname patterns for -W

git-grep avoids loading the funcname patterns unless they are needed.
ba8ea74 (grep: add option to show whole function as context,
2011-08-01) forgot to extend this test also to the new funcbody
feature.  Do so.

The catch is that we also have to disable threading when using
userdiff, as explained in grep_threads_ok().  So we must be careful to
introduce the same test there.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>