Focus commit input box after SCM commit action

[mattseddon]

From talking with Alexander yesterday I think that we want to cut down the number of clicks / actions to get users through particular workflows.

In the SCM view we currently have a commit button which runs dvc commit (for the entire repository) but then stops. In this PR I have made the cursor jump to the git commit inputBox once the user has dvc committed their DVC changes. This should serve to educate users that they still have to enter a commit message and commit to git once they have run dvc commit.

Thought process for this going with this approach:

1. It is fairly simple.
2. We don't have to clutter the UI further with our own input box (we had this previously and it was confusing IMO).
3. We do not need to run any git actions in the background.
4. We are integrating with something that is already there.
5. Works well for the happy path.
6. User can still use the UI to stage some files and commit only those if they want to.
7. If the user chooses to enable the git.enableSmartCommit option then it is fairly quick.

IMO if you watched a 30 second video explaining the flow it would make the whole process / underlying DVC semantics more intuitive. LMK what you think.

Demos

Without git.enableSmartCommit (this can be enabled by clicking "always" from the second popup):

Screen.Recording.2021-09-30.at.11.16.13.am.mov

With git.enableSmartCommit:

Screen.Recording.2021-09-30.at.11.17.46.am.mov

Relates to #609.

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