feat(squash): Add --keep-emptied flag

[matts1]

If the user was thinking in terms of files, instead of commits, then we shouldn't abandon the commit, as that can have side effects such as moving branch pointers.

Fixes #3815

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• [N/A] I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[mlcui-corp]

Thank you for implementing this! A few drive-by nits.

[yuja]

Thanks for updating the patch. This looks good to me.

[matts1]

One last thing I'd like to discuss before submitting is what the name of the flag would be. I'll wait for a few days before submitting for any feed back on if people want the name changed.

Our options seem to be (feel free to add more if you have any ideas):

• keep-emptied
  • The most technically accurate mapping of behavior to name
  • Is "keep" going to be the term we always use in the future for "don't abandon". I'd like to be consistent about this.
• keep-empty
  • Technically, this would imply that we abandon all empty commits, which isn't correct in the one edge case where we squash two empty commits together.
• no-abandon
  • Would imply that the default is abandon (in reality, the default is abandon empty commits)
• never-abandon
  • Doesn't have the downsides of the implications of --no-abandon
  • A less common way of phrasing it
    • May be confusing, or intentionality may provoke thought and understanding of what it actually does.