First, check the brew update upgrade information

brew update –verbose –force

fatal: Needed a single revision

Previously modified the source, resulting in 416, GIT update failed

Reset all sources

Reset brew.git:
cd "$(brew --repo)"
git remote set-url origin https://github.com/Homebrew/brew.git

Reset homebrew-core.git:
cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
git remote set-url origin https://github.com/Homebrew/homebrew-core.git

brew update –verbose –force

then

Updating /usr/local/Homebrew/Library/Taps/caskroom/homebrew-cask...
fatal: Needed a single revision

need to goto /usr/local/Homebrew/Library/Taps/caskroom/homebrew-cask

git pull 
From https://github.com/Caskroom/homebrew-cask
 * branch                  HEAD       -> FETCH_HEAD

Re-execute brew update to restore normal.

Continue brew search opencv2

to find 2 libraries normally.

brew install opencv@2