fugitive compatibility for viewing commits in PlugDiff

In `:PlugDiff`, it's possible to hit enter on a given commit to view its content:

[fugitive.vim](https://github.com/tpope/vim-fugitive/) has a very similar feature for viewing commits, but with extra features like being able to browse to other commits and files, and with [rhubarb.vim](https://github.com/tpope/vim-rhubarb/) and others, to open the equivalent forge URL e.g. on GitHub.

It'd be cool if vim-plug used fugitive to view commits when it's available.

```
NVIM v0.9.5
Build type: Release
LuaJIT 2.1.1703358377

system vimrc file: "$VIM/sysinit.vim"
fall-back for $VIM: "/opt/homebrew/Cellar/neovim/0.9.5/share/nvim"
```

- Type:
- [ ] Bug
- [x] Enhancement
- [x] Feature Request
- [ ] Question
- OS:
- [x] All/Other
- [ ] Linux
- [ ] macOS
- [ ] Windows
- Vim:
- [x] Terminal Vim
- [x] GVim
- [x] Neovim