Move 'last commit' and amend support so still there when using git vscode built-in

[westbury]

Description

We should move the git amend support into git-extra package and ensure it works with both @theia/git and the built-in. The SCM-independent part of the amend support should be moved to scm-extra, with the SCM view having support for contributions to the bottom of the view.

This work follows on from #6381

Reproduction Steps

Remove @theia/git from the build and drop in the Git built-in plugin. You will see that the 'last commit' section at the bottom of the SCM view has disappeared. The 'last commit' section should be there the same regardless of whether @theia/git or the plugin is being used.

[westbury]

I plan on working on this after #7505 . That PR converts the parts of the Source Control view to widgets, so then this issue can be solved by allowing downstream packages to contribute widgets to the bottom of the Source Control view.

The uncertainty I have, however, is that we will have both the Git built-in and Theia's Dugite code both accessing the same repository. I have not yet tested to see if this creates issues. The built-in does not expose what we need to do the 'amend' suport.

[vince-fugnitto]

@westbury any news on the issue? I believe it is the last remaining blocker for #7482.

[westbury]

@vince-fugnitto I've had the PR opened and ready to go for a while. I too was wondering about the state of this, and whether there is still interest in moving to the built-in. We still have concerns about using the built-in, which generally come down to the fact that we can't configure it. Maintaining our own fork of the Git built-in is something we would really like to avoid.

In any case I need to rebase the PR so I will do that.