Also makes it the default

Adds advanced.externalDiffTool setting
Adds advanced.externalDirectoryDiffTool setting

Adds repo change events for rebase, merge, and cherry-pick
Clears some caches only when specific change occurs

Moves a few api props to be at the root level

fs.realpath.native in NodeJS uses the Win32 API function GetFinalPathNameByHandle() on Windows hosts,
therefore a given path must follow the guidelines for Win32 API file functions.
Drive letters on Windows need to end on a backslash according to the Win32 File Naming Conventions (https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file)
Omitting the backslash results in Windows treating the remaining path components as a relative path starting from the current directory on the specified disk
https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file#fully-qualified-vs-relative-paths

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.