Allow setting a default name when creating new branches

[elliotcubit]

• PR Description

I commonly prefix my branch names with my first initial and last name, such as this one (ecubit/branch-prefixes). It can be a bit annoying to type out.

This PR adds a config option to set a default value for the name in the branch creation modal.

If there would have previously been a branch name autofilled (I do not know all such cases), this change has no effect.

• Please check if the PR fulfills these requirements

• [x] Cheatsheets are up-to-date (run go generate ./...)
• [x] Code has been formatted (see here)
• [x] Tests have been added/updated (see here for the integration test guide)
• [ ] Text is internationalised (see here)
  • CONTRIBUTING.md says I may submit without doing localization (and I am unable)
• [x] Docs (specifically docs/Config.md) have been updated if necessary
• [x] You've read through your own file changes for silly mistakes etc

[EliasA5]

why not make it repository specific as well, like in predefined commit message prefix.

[jesseduffield]

Thanks for making this @elliotcubit . I've taken the liberty of pushing a change to the test to make it more focused.

@EliasA5 we'll eventually have a proper solution for defining arbitrary repo-specific config values, so I'd rather wait for that than to add another thing like commitPrefixes now. See https://github.com/jesseduffield/lazygit/issues/3293