feat: Add --diff filter

[olexale]

Description

We found it extremely helpful to have the possibility to run melos on packages that are different between branches. As the since option does not really work with the difference I propose to introduce a diff option that will call git diff instead of git log.

The usage might be:

melos list --diff=some_branch

or

melos list --diff=origin/main...HEAD

Type of Change

• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• ❌ ! -- Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 refactor -- Code refactor
• ✅ ci -- Build configuration change
• 📝 docs -- Documentation
• 🗑️ chore -- Chore

[CLAassistant]

All committers have signed the CLA.

[blaugold]

Thank you for your contribution! 🎉

Could you add documentation for the new option in the docs, with examples for a simple ref and ranges?

@Salakar
--diff=<ref>...HEAD is equivalent to --since=<ref>, so we could deprecate --since in favor of --diff.

[olexale]

@blaugold, done, please review.

[blaugold]

I've made diff and since mutually exclusive. Otherwise, LGTM.