[vim keymap] Fix zz/zt/zb not working

@mightguava
Seems like the unit tests does not cover this set of features.

When I press `zz``zt``zb`, the text doesn't scroll for me.
But when I run `CodeMirror.Vim.handleKey(cm, 'z')` twice, the text scrolls as expected.
Pretty weird.

With a little inspection, I figure out it's caused by `cm.scrollIntoView()`.

The current patch uses some minor tweaks to work instead of `cm.scrollIntoView()`.