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

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>