grep: enable threading with -p and -W using lazy attribute lookup

Lazily load the userdiff attributes in match_funcname().  Use a
separate mutex around this loading to protect the (not thread-safe)
attributes machinery.  This lets us re-enable threading with -p and
-W while reducing the overhead caused by looking up attributes.

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