The short answer is, you cannot search commit messages directly on github.com the website. For the time being we recommend the local git grep
solution others on this thread have proposed.
At one point in time GitHub did offer a git grep
style search over commit messages for a single repository. Unfortunately, this approach exposed a denial of service that could render a file server inaccessible. For this reason, we removed git grep
searching.
Current back-of-the envelope estimates puts the number of commits in GitHub somewhere around the 80 billion mark. Although Google engineers laugh behind our backs, this is a rather large number of documents to store in ElasticSearch. We'd love to make this dataset searchable, but it is not a trivial project.