The following URL will show all the commits for a single file in a format similar to git log -p:
http://github.com/<username>/<project>/commits/<branch>/<path/to/file>
…where:
<username>is the username of the person that owns the repo<project>is the repo name<branch>can be ‘master’ or any other branch<path/to/file>is hopefully self-explanatory
Picking at (somewhat) random, here is an example from the vim-fugitive repo.