Add :Gtdiff, :Gtsdiff, :Gtvdiff, and related mappings

Upon further reflection,  I concluded that diffs in a new split are a little too silly. Diffs in a new tab, though, still seem like a good idea, so I added it. This closes issue #318

- `:Gtdiff` is `:Gdiff` but in a new tab
- `:Gtsdiff` is `:Gsdiff` but in a new tab
- `:Gtvdiff` is identical to `:Gtdiff` (for symmetry with `:Gtsdiff`)
- `dt` in the status window does `:Gtdiff`
- `dts` in the status window does `:Gtsdiff`
- `dtv` in the status window does `:Gtvdiff`