Upgrade grep to 2.20

grep-2.14 has a bug with Unicode when calling mbrtowc() on a
wide character that won't fit into a single wchar_t.  grep-2.20 has
a fix for this (Cygwin had this problem originally), so we will
bring up to their latest version.

Signed-off-by: Doug Kelly <dougk.ff7@gmail.com>