Modern homebrew links cellar in /usr/local/opt too

[sandercox]

Was getting errors on git installs using brew pkg. Complaining it couldn't find templates. Added a creation of /usr/local/opt/<pkg> to the latest cellar version too in the script fixes the issue.

[sandercox]

I noticed that this package got an update after a long time. After my brew updating this crashed the brew-pkg we create (nightly) for the latest git version (if it changes). Particular git needs the links from this PR to make sure they can be deployed properly.
Our CI server starts to complain when trying to checkout git repos with:

git: 'remote-https' is not a git command. See 'git --help'.

After I manually link /usr/local/opt/git to /usr/local/Cellar/git/ it works again. (Just what this patch fixed)

Would be nice if it could be included in a next release of brew-pkg! We still use it all the time!