Install w/ --HEAD
Formulae can specify alternate downloads for the upstream project’s devel release (unstable but not trunk) or HEAD (master/trunk).
HEAD URLs (activated by passing --HEAD) build the development cutting edge.
Adding a --HEAD flag to brew reinstall will likely interact poorly when having both stable and head installed.
If you did brew install --HEAD food, then brew reinstall foo will reinstall foo from the latest commit. There is no need to pass --HEAD to brew reinstall.
# Install isync from HEAD
brew unlink isync
brew install --HEAD isync
# Revert
brew reinstall isync