Add custom mappings and settings for `git` files

- For git logs: `<C-N>` and `<C-P>` jump to the next and previous
  commits, respectively, aligning the beginning of the commit to the top
  of the window.

- Set `'scrolloffset'` to `0` so aforementioned aligning is possible
Remove whitespace from the end of git mappings

- There was whitespace between the end of the mappings and the bars that
  identified the end of the statement; those are interpretted literally,
  causing the mappings to end with the cursor moving forward one space
Update plugins