Add index to unstage command

[colin-grant-work]

What it does

Fixes #11633

How to test

1. Open a Git repository.
2. Modify several files.
3. Stage one or more of them.
4. Unstage one of the staged files using the SCM UI.
5. That single file should be unstaged, other staged files should remain staged, and the content of no file should be changed.

---

1. Try commands in the SCM widget at different levels (Resource Group, Folder)
2. Confirm that they behave as expected.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[jaredkipe]

I could be wrong, but there are multiple this.git.unstage(repository, uris) calls in the file that seem like they may benefit from this solution. I tried patching the file in Docker but I was unsuccessful without just reverting the library itself. So I cannot 'review' it as working.

[vince-fugnitto]

I could be wrong, but there are multiple this.git.unstage(repository, uris) calls in the file that seem like they may benefit from this solution.

@jaredkipe there is only one other git.unstage call I can identify and it correctly uses the default all option to unstage all:

theia/packages/git/src/browser/git-scm-provider.ts

Lines 318 to 326 in a07b746

	async unstageAll(): Promise<void> {
	try {
	const { repository, stagedChanges } = this;
	const uris = stagedChanges.map(c => c.uri);
	await this.git.unstage(repository, uris);
	} catch (error) {
	this.gitErrorHandler.handleError(error);
	}
	}

[vince-fugnitto]

I confirm it works as expected 👍 I'll give @msujew a chance to review as well.

[msujew]

Looks good to me 👍 I can confirm that the issue exists on master and is addressed by this change.