Git Bisect: Find the commit that broke my tests

So master is broken and you don't know which commit contains the offending code....

Git bisect is an amazing tool. It allows you to flag which commit was the last green state, and automatically iterate over each subsequent commit, running a script (i.e. your tests) until it finds the breaking commit....

i.e.

git bisect start [some-breaking-commit-sha] [last-good-commit-sha]
git bisect run [test-command]

a working example using rspec:

git bisect start 120cba5 c2d0a5e
git bisect run rspec spec/mytest_spec:123

Git: patching diffs between branches

Recently I accidentally committed changes to master that I want to commit in a feature branch..... I made the mistake of

1. branching master (to a feature branch) from my erroneous commit - then
2. resetting (HARD) master to the previous commit

This axed all my history in my feature branch as the commit which I branched from in master no longer existed....

To fix I had to create a new feature branch from master, then create a patch of the differences between my original feature branch and master doing the following:

git checkout master
git checkout -b [new_feature_branch]
git diff --no-prefix origin/[busted_feature_branch] > my.patch

then apply the patch:

patch -p0 < my.patch