gitk feature request: --goto

I was just writing beginner-level instructions for looking at a specific
commit in gitk ('double-click on the "SHA1 ID" box to selet the contents,
type "5401f304" in its place, and click "goto"'), and it dawned on me that
it would be easier to look at a given commit in context if there was a
--goto <commit-ish> option as well as the usual filtering options.

It might be a nice feature for "git bisect visualize" to preselect the
currently-being-tested commit, for example, rather than making the user
look for the "bisect" head.

And it seems like such a trivial thing to add, I figured I'd ask.

(You can add some highlight options as well, if you feel ambitious,
although I can't think of a use for them right now.)


Oh... when we add the option to have HEAD point to a non-head, it might
be nice to add a green box or other symbol for HEAD, too.  Just one more