Prevent pushes that break the build.

pre-push

# Requires git 1.8.2 or above.  This excludes `git` from Xcode 4.6, but should include `git` from Xcode 5 and homebrew.
# Not perfect. If you've done anything since your last commit that breaks the build, this will incorrectly fail. Maybe do something like https://twitter.com/drance/status/354365644147392513 + https://twitter.com/cbarrett/status/354372047062835201)?
# Install by adding this file into .git/hooks, naming it `pre-push`

#!/bin/sh

# go into the dir that contains the xcodeproj
cd Foo

# build the xcodeproj for your scheme (and/or configuration)
xcodebuild -scheme Foo

# if we built successfully, exit 0 and push
if [ "$?" = 0 ]; then
  exit 0
fi

# otherwise, exit non-0 and prevent the push.
exit -1