Move to SF Symbols style icons

[Uncommon]

Currently the icons in the UI come from, or are based on, GitHub's octicons. But with the advent of SF Symbols, it looks like a good idea to move to that style. I like having a standard Mac UI, and SF Symbols has several icons that will be useful.

Icons that will need to be made:

• [ ] Fetch, Push, Pull (maybe)
• [ ] Branch
• [ ] Stage/unstage all
• [ ] Pull request
• [ ] Staging
• [ ] Diff
• [ ] File status: (maybe)