Add support for keyboard shortcuts on multi-selection menus

Nobody has claimed this yet.

Currently, running `git cm` offers a quite lengthy menu:

```
$ git cm
? Select the type of change that you're committing: ›
❯ feat: A new feature
test: Adding missing tests or correcting existing tests
ci: Changes to our CI configuration files and scripts (example scopes: Travis, Circle, GitHub Actions)
build: Changes that affect the build system or external dependencies (example scopes: cargo, bazel, make)
style: Changes that do not affect the meaning of the code (white-space, formatting, etc)
chore: Other changes that don't modify src or test files
perf: A code change that improves performance
fix: A bug fix
revert: Reverts a previous commit
docs: Documentation only changes
refactor: A code change that neither fixes a bug nor adds a feature
```

Being able to instantly select one with a key-press would make it really faster to use.

I'm thinking about some letter hints, which are indicated by a line below the relevant character.

Start at the `git cm` entry point and trace how the multi-selection menu receives keyboard input. Confirm how menu items are represented and how selection is tested or manually exercised. Done means letter hints are shown for the relevant characters and pressing a hint immediately selects the corresponding commit type.

Written by the indexing model from the issue text.