Git bisect is a powerful automated tool for searching deep into a project's history. Instead of searching for relevant commit messages
(git log) or patches
(git log -S), bisect actually allows you to run a functional test on each revision until the first bad commit is identified. (Okay, it doesn't test every revision, it performs a binary search, which results in at most log2(N) tests. This allows a relatively large history to be searched quickly.)
The test can be done interactively, with the human performing each check, or mechanically if you can supply a testing script. Randy Fay has done a nice screencast on the interactive method; this post will instead focus on mechanizing the process.
For an example, let's look at a core Drupal bug that impacts this very site: #812990: Search page title changes to Home. For the moment, we'll pretend the cause of this bug isn't already known, and hunt it with git-bisect.