Update fugitive mappings

* Use `<C-G>` as leader
* Add mapping for `:Git log`
* Change mapping for `:Gllog` to `<C-G><C-F>` / `<C-G>f`
* Clean up `:Gdiff` and `:Gedit` mappings, make both accept count
* Clearer comments
Update `'fillchars'` and `'listchars'`