add 'diff in new tab'

I've already searched and read postings related with "diff in tab page" topic ([Issue #36](https://github.com/tpope/vim-fugitive/issues/36), [Issue #76](https://github.com/tpope/vim-fugitive/issues/76), [Pull Request #319](https://github.com/tpope/vim-fugitive/pull/319)).

I don't think that you prefer doing diff in a tab page.
But I think not a few people want to do diff in that way, because diff in a tab page has several advantages :
- Maximized diff window would be helpful for comparing two files, especially for long files. Even for short files, one can compare two files at a first glance.
- Diff in that way does not have any influence on the original working buffers / windows in vim. After diff, one can return to their original workspace just by closing the diff tab page.

I added only two features, so I think that the commands are not getting messy and still clear.
- 'dt' in Gstatus window
- :Gtdiff command

This is my first pull request, so please understand me if I make some mistakes.