Unit tests for the field should probably be added (mentioned as a "TODO" in your Testing Done).
Did your test run include JavaScript unit tests, and review requests without history support?

I'm switching more code over to this (I have a pending change I need to split up and put up for review), but for render_to_string() you'll want to use the new compat function we have, which will help us migrate to newer versions of Django soon.

from djblets.util.compat.django.template.loader import render_to_string

...

render_to_string(template_name=...,
                 context={...},
                 request=request)


Passing in request gives you a RequestContext. If not passed in, you get a Context.
(It does support passing in those objects themselves, but a dictionary is more future-proof.)

We do this both here and in should_render(), but we do it differently in each. We should be consistent, or we should have a private helper that wraps it.

Typo: "rquest" -> "request"

Rather than loop through the commits multiple times, how about:

include_author_name = not submitter_name
should_expand = {}
show_expand_collapse = False

for commit in commits:
    commit_should_expand = (commit.summary.strip() != commit.commit_message.strip())
    should_expand[commit.pk] = commit_should_expand

    if commit_should_expand:
        show_expand_collapse = True

    if not include_author_name and commit.author_name != submitter_name:
        include_author_name = True


Potentially, you could also get rid of show_expand_collapse by turning should_expand into a set of ones to expand. It's then easy to check against that for both individual commits and as a larger "are there any commits that should expand". Then we're simply looking at:

include_author_name = not submitter_name
should_expand = set()

for commit in commits:
    if (commit.summary.strip() != commit.commit_message.strip()):
        should_expand.add(commit.pk)

    if not include_author_name and commit.author_name != submitter_name:
        include_author_name = True

This sounds like a boolean, but it's a dictionary. Maybe call it commits_to_expand?

Let's make this a private method on the class, so it's not redefined every time this method is called.
Alternatively, do this instead:

return {
    key: set(
        {
            'author': commit.author_name,
            'summary': commit.summary,
        }
        for commit in commits_by_diffset_id[info[key]]
    )
    for key in ('old', 'new')
}

The annoyance with {} for both sets and dicts is that this looks pretty weird at first glance. Thinking we should use set() explicitly here to differentiate from the dict.

Missing trailing comma.

We should probably explicitly check that diffset_id is not None here.

Missing a docstring.

Typo: "creatd" -> "created"

We can define this once in the base class instead of in each subclass by doing:

if not self.get_review_request().created_with_history:
    return None

if not hasattr(self, '_commit_messages'):
    self._commit_messages = list(
        self.get_latest_diffset().commits
        .values_list('commit_message', flat=True)

return self._commit_messages

Typo: "creatd" -> "created"

Unwanted blank line.

Alphabetical order.

Can you define a constant for this?

We should either have a constant already for the font size, or you should be able to inherit.

Can you define a constant for this?

We should have constants for this already, since we have the same styling elsewhere. If not, it should be added and used both here and in the file list.

Blank line here.

This is missing the mobile rule in #review-request-main. Really, we should add a mixin that's used both here and there that sets up this styling and the mobile styling. Probably want to sneak in the position: relative and display: block as well, I'd think.

Comma before "if any."

Here as well.

Looks like you got some whiteout on your '.

Typo: "statis" -> "status"

Typo: "Collpase" -> "Collapse"

You can do this.$('.commit-message') instead.

Can you start the first parameter on the function call line? That or turn this into a dictionary. Looks a bit out of place the way it is.

Typo: "colum" -> "column"

Instead of - and +, can you use fa fa-plus and fa fa-minus? That's what we use for other collapse/expand controls.

This should be the new full type path.

You should use keyword arguments for these calls to render_to_string(), to ensure compatibility going forward.

Can you put the start of the tuple on the second line?

"Array", not "array".

"Backbone", not "Bckbone".

Alphabetical order.

RequestFactory() should be defined in setUp and not setUpClass, since it keeps state between requests and can introduce subtle failures.

Should be Model Attributes.

Should be jQuery.Event.

Should be jQuery.Event.

Should be jQuery.Event.

Should be boolean.

Ship It!