macOS pkg installer: fix existing installation not being upgraded

[EricFromCanada]

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Currently when running the macOS installer atop an existing installation, at the point after the installation finishes but before postinstall is run, the master branch hasn't had its HEAD advanced to the latest tag yet so the working directory is in a "HEAD detached at {version number}" as though you had run git checkout {version number}. This is why git reset --hard resets it to the previous tagged version.

This change has git find the latest tag (git tag --list --sort="-version:refname" | head -n1) and then resets to that tag (git reset --hard <tag name>). This also adds a git clean -f -d to clear any stray untracked files from the working directory, and no longer purges all branches other than master.

Fixes #19287, alternate to #19354. Tested on both macOS ARM and Intel.

[ZhongRuoyu]

Thanks!