gitk: Pass --invert-grep option down to "git log"

"git log --grep=<string>" shows only commits with messages that match the given string, but sometimes it is useful to be able to show only commits that do *not* have certain messages (e.g. "show me ones that are not FIXUP commits"). Now the underlying "git log" learned the "--invert-grep" option. The option syntactically behaves similar to "--all-match" that requires that all of the grep strings to match and semantically behaves the opposite---it requires that none of the grep strings to match. Teach "gitk" to allow users to pass it down to underlying "git log" command by adding it to the known_view_options array. Signed-off-by: Christoph Junghans <ottxor@gentoo.org> Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Paul Mackerras <paulus@samba.org>