Add support for downloading formula and cask binaries in the background

[metametapod]

Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

It would be cool if there were a way to automatically download formula and cask upgrades in the background without installing. E.g. something like:

brew upgrade --download --cask transmit

Then when running brew upgrade later, it can prefer the cache if it's still up-to-date.

What is the motivation for the feature?

Currently there is only support for brew autoupdate start --upgrade, but that installs the update which can cause incompatibilities.

How will the feature be relevant to at least 90% of Homebrew users?

Adding an option to download would speed up Homebrew considerably especially for those on limited bandwidth connections, while still allowing one to vet updates before installing.

What alternatives to the feature have been considered?

As far as I'm aware there is no alternative available, other than running brew update which does not download binaries, or brew autoupdate start --upgrade which both downloads and installs the update.

[MikeMcQuaid]

Then when running brew upgrade later, it can prefer the cache if it's still up-to-date.

brew fetch --cask transmit should do this.

[carlocab]

If you want it to be done in the background:

brew fetch --cask transmit &

[metametapod]

Thank you, I didn't know about brew fetch. To prefetch all pending updates (both formulae and casks):

brew outdated | awk '{print $1}' | xargs brew fetch --deps