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

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