Update Operator to get contents of commit files

[sahansk2]

We'd like to have a common method to get the contents of a file in a commit, something like

def commit_content(self, commit, filepath)

since this is pretty crucial for level development in #238.

commit could support taking a Commit object, or a string commit-ish.

filepath should support taking Path objects and string representations of paths.

[benthayer]

This is pretty similar to the implementation of #286 .

We should also be wary of reading the data stream, which can only be read from once.