Add commit fixup and rebase with autosquash

[bkmeneguello]

Issue Number / Link:

closes #410

Summary of the issue:

Adds an option to create a fixup commit and also adds an option to autosquash interactive rebasing.

[bkmeneguello]

Hi @mhutchie, any chance you check this? I use this feature daily basis and would be awesome to have it in vscode. Also, I have more like this to add if I could get it approved.

[Ghostbird]

@mhutchie I'd love to see this merged, do you have time to have a look at this?
@bkmeneguello I see some issues with this PR (keep in mind, I've no power here, I'm just a user who'd like this feature)

1. You have enabled vigilant mode but have not signed this commit.
2. It is actually two separate features in one
   • Control over the autosquash flag on rebase.
   • A fixup commit context menu option.
3. I think the autosquash doesn't account for git config

Personally I don't care for the autosquash option, and I don't see it load the initial value from git config, nor override it. I think the actual logic executed may differ significantly from what the user selects. I don't know many use cases for not autosquashing, I always enable it by default system-wide. I've performed thousands of rebases and hundreds of fixups in the last few years, and I don't remember ever having to clear the fixup set by default autosquash.

The fixup context menu entry really adds something significant though. It's the one command that I regularly use that I have to run from the terminal.

In conclusion, I'd suggest splitting the features of this PR across two separate ones. Because the fixup option in my opinion is more desirable, and easier to merge.

[ShuXiao2023]

This is really a great feature, and I hope this can be merge too~~