When available via the built-in Git api

From February 2021, in order to provide feedback on pull requests, Code Scanning workflows must be configured with both `push` and `pull_request` triggers. This is because Code Scanning compares the results from a pull request against the results for the base branch to tell you only what has changed between the two.

Early in the beta period we supported displaying results on pull requests for workflows with only `push` triggers, but have discontinued support as this proved to be less robust.

See https://docs.github.com/en/free-pro-team@latest/github/finding-security-vulnerabilities-and-errors-in-your-code/configuring-code-scanning#scanning-pull-requests for more information on how best to configure your Code Scanning workflows.

Previous commit a664073

Works around https://github.com/microsoft/TypeScript/issues/41428

eamodio/vscode-gitlens#611 added a setting for commit SHA length,
`gitlens.advanced.abbreviatedShaLength`.

This commit will add `gitlens.advanced.useAbbreviatedShaLengthForCopy`,
which will copy the abbreviated commit SHA instead of the full SHA.
Respects `gitlens.advanced.abbreviatedShaLength`.

Adds @kh0m contribution

Related Issue: #1306

Adds caching to merge/rebase status

CommitsFileNode = a file under a commit (shown as a file)
FileRevisionAsCommitNode = a file revision shown as a commit