feat(fugitive): line position tracking for keymaps

When pressing `du`/`dU` from a hunk line in the fugitive status buffer
(after expanding with `=`), the unified diff now opens at the
corresponding line instead of line 1.

Implementation:
- `fugitive.get_hunk_position()` returns @@ header and offset when on a hunk line
- `commands.find_hunk_line()` finds matching @@ header in diff buffer
- `commands.gdiff_file()` accepts optional `hunk_position` and jumps after opening

Also updates @phanen's README credit for the previous two fixes.

Closes #65

README.md

@@ -69,4 +69,5 @@ luarocks install diffs.nvim
 - [`vim-fugitive`](https://github.com/tpope/vim-fugitive)
 - [`codediff.nvim`](https://github.com/esmuellert/codediff.nvim)
 - [`diffview.nvim`](https://github.com/sindrets/diffview.nvim)
-- [@phanen](https://github.com/phanen) - diff header highlighting
+- [@phanen](https://github.com/phanen) - diff header highlighting, unknown
+  filetype fix, shebang/modeline detection