How to git bisect in projects using Bundler

A few things that may not be obvious are needed for git bisect to work in a project that uses Bundler.

1. The Gemfile.lock needs to be in the git repo, so that each commit will load the same dependencies every time.
2. Each step during the bisect needs to run bundle install first, so that the correct dependencies are installed and available to be loaded.
3. After determining if the commit is good or bad, each step needs to git reset. If bundle install or running the test can cause changes on the file system, which would prevent git checkout of the next commit to test if they are not reset.

Here’s a minimal example script that runs the rake task spec:

#!/usr/bin/env bash
bundle install
bin/rake spec
status=$?
git reset --hard HEAD
exit $status

See also the discussion at bundler/bundler#3726.