[git] Add missing items in README.md
[git] Add missing items in README.md

namely, the `gfu` alias, the `git-branch-delete-interactive` function,
and a couple of commands that are shadowed.

Command `gm` was reported in #59, and `grc` was reported in #139.

joshuarubin/dotfiles,joshuarubin/dotfiles,Eriner/zim,joshuarubin/dotfiles,joshuarubin/dotfiles