Future twiggy conflicts

I have some major refactoring for twiggy.vim on the horizon. One issue is that I no longer want to use `^` and `V` for pushing and pulling. I was always a fan of `^` until I started working at a place that uses really long branch names—I got my co-worker to try out twiggy and he hit `$` to read the rest of the branch name then `^` to get back but it pushed! I also don't like to use mappings that still make sense in unmodifiable buffers (`^` and `V` are the only two that violate this).

TL;DR, I am soon going to be deprecating those mappings in favour of `P` and `p`.

To keep in line with twiggy as well, I really like to have just one character mappings. I was trying to think of a good one for opening a PR. The best I can come up with is `X` (as it stands, lowercase is for local stuff and uppercase is for remote stuff).

I'm also softening to the idea of just including this in twiggy if you're still open to it.

Start by reviewing the issue discussion and the existing twiggy.vim mappings. The proposed work is to decide and implement replacements for the ^ and V mappings and choose an X mapping for opening a pull request; no specific file or test is named, so the scope needs clarification before coding.

Written by the indexing model from the issue text.