问题：
brew install tree
fatal: Could not resolve HEAD to a revision
==> Searching for similarly named formulae...
Error: No similarly named formulae found.
Error: No available formula or cask with the name "tree".
==> Searching for a previously deleted formula (in the last month)...
Error: No previously deleted formula found.
==> Searching taps on GitHub...
Error: No formulae found in taps.
解决:
rm -rf $(brew --repo homebrew/core)
brew tap homebrew/core
