Would it be possible to split out the ES6 port part of this change into a separate review request?

request -> requested?

request -> requested?

There's a lot of nesting and conditionals in here.
Can we split this up to be more readable?

Too many blank lines now.

Needs a trailing comma.

Needs a trailing comma.

Missing the explanations here.

Needs a doc comment.

Looks to me like this returns a function, not an object.

Can we use a ~ for each class reference to trim the output a bit?

Let's get rid of the double loop and need for constructing generators. What do you think of:

needs_base_commit = base_commit_id is not None
needs_tip_commit = tip_commit_id is not None

if needs_base_commit or needs_tip_commit:
    for commit in commits:
        if needs_base_commit and commit.pk == base_commit_id:
            base_commit = commit
            needs_base_commit = False

        if needs_tip_commit and commit.pk == tip_commit_id:
            tip_commit = commit
            needs_tip_commit = False

        if not needs_base_commit and not needs_tip_commit:
            break

Too many blank lines.

There's some redundancy in handling of missing data here. We know index is in GET, but we call .get() on it anyways. We're also then doing exception handling anywya. How about instead we do:

try:
    diff_file['index'] = int(self.request.GET['index'])
except (KeyError, ValueError):
    pass

string

string

Can you put parens around the expression?

These are indented too far.

These should all align.