* Don't infinitely recurse `brew update --preinstall`.

This could happen when trying to `brew install git` inside `brew update
--preinstall`.

* update.sh: cache Git PATH.

We don’t need to look it up from superenv every time; this is slow.

* update.sh: print message before preinstall updates.

* update.sh: verbose output fetch directory.

This aids reading `brew update --verbose --debug` output.

* update.sh: skip taps without formulae on preinstall.

We don’t need to update them as we’re not invoking them.

* update.sh: don't force update-report on developer preinstall.

This is too slow.