fix: self-update check for updates

[vladdoster]

Motivation and Context

Match Homebrew's behavior

~/.local/share/zinit/zinit.git main*
MDN-X5J9PHV9NQ ᐳ brew update
Already up-to-date.

Usage examples

Current behavior

~/.local/share/zinit/zinit.git main
MDN-X5J9PHV9NQ ᐳ zi self-update
[self-update] fetching latest changes from main branch
Already up to date.
[self-update] compiling zinit via zcompile
[self-update] reloading zinit for the current session

New Behavior

~/.local/share/zinit/zinit.git main*
MDN-X5J9PHV9NQ ᐳ zi self-update
Already up-to-date.

How Has This Been Tested?

Types of changes

• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Documentation change
• [X] New feature (non-breaking change which adds functionality)

Checklist:

• [ ] All new and existing tests passed.
• [ ] I have added tests to cover my changes.
• [ ] I have updated the documentation accordingly.

[vladdoster]

@pschmitt,

This is the first applicable use of this function, but can be used in a few more places to improve logic.

[vladdoster]

@pschmitt,

This PR is ready for review.

[github-actions[bot]]

:tada: This PR is included in version 3.12.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: