Closes #2959 autosquash commits in interactive rebases

[gabeseltzer]

Description

Closes #2959

Adds the --autosquash flag to interactive rebases.

Note, I wasn't 100% sure if this is the right solution, but it works and was easy. As far as I know, there's no real downside to always using the autosquash feature. I had some other ideas but they were harder so I thought I'd PR and see what people thought. Here's the other options:

1. Simply add --autosquash to the arguments for the existing --interactive line item for everyone. <<< This is what I did
2. Simply add --autosquash to the arguments for the existing --interactive line item if users enable it with a vscode extension setting
3. Add a new option that does both --interactive --autosquash, and show it all the time
4. Add a new option that does both --interactive --autosquash, and only show it if there are fixup commits in your rebase range

Checklist

• I have followed the guidelines in the Contributing document
• My changes follow the coding style of this project
• My changes build without any errors or warnings
• My changes have been formatted and linted
• My changes include any required corresponding changes to the documentation (including CHANGELOG.md and README.md)
• My changes have been rebased and squashed to the minimal number (typically 1) of relevant commits
• My changes have a descriptive commit message with a short title, including a Fixes $XXX - or Closes #XXX - prefix to auto-close the issue that your PR addresses

[gabeseltzer]

@eamodio sorry to ping you directly, but I wasn't sure if there was a proper way to set a reviewer, etc. Thanks!