Avoid clobbering the original buffer's alternate file.

Opening of the Undotree shouldn't affect CTRL-^ nagivation to the original alternate file, `:keepalt` fixes that.

To reproduce this, open `fileA`, then `:edit fileB`. Now open the `:UndotreeShow`. In the original buffer, `:ls` should still show the `#` sigil before `fileA`, not before `undotree_2`.

PS: Thank you for including my other two pull requests!