Uses the core git fetch command instead of a command-line
git fetch so VsCode will prompt the user for their username
and password. Pull & push also use their corresponding core commands.

In case of multiple cursors (selections) in the same line, an
annotation was created per selection, resulting in an odd experience.
Filter out duplicated lines by the line number.

Add author to CHANGELOG and README

Fixes: #2346

- updates README
- minor updates to GL+ walkthrough intro

- Adds the Commit Graph
- Adds the Commit Details view

- adds command palette action

Adds line fading option to heatmap annotation

And separates the existing Gerrit provider to a new GoogleSource one.

Closes #1953

Honors `git.repositoryScanMaxDepth` if not overridden