Can you wrap your description at 72 characters instead of 100+?
Same for testing done.

Can you add a screenshot of the rendered HTML e-mail view with the new content?

I know this is a small change, but can you take a peek at our guide for writing good change descriptions. Specifically, the description and testing done should consist of complete sentences.
Also the description should read more like

E-mails will now contain the contents of the branch field ....

i.e., the sentences should be descriptive not summaritive.

We probably want these one right after the other. Unfortunately, the Django templating language is kinda messy when it comes to newlines (since it's really built for HTML where it rarely matters). Each newline in there, including the ones after the template tags, will be included in the plain-text e-mail. That means you're going to get something like:

Repository: my-repo



Branch: my-branch


Description
-----------


(This is only a problem in plain-text e-mails.)
Instead, we'll want to collapse those. The good news is, we have a handy tag you can use. called condense. This would look like:

{% condense 0 %}
{%  if review_request.repository %}
Repository: ...
{%  endif %}
{%  if review_request.branch %}
Branch: ...
{%  endif %}
{% endcondense %}

Description.....


Make sure you run the preview-email view with this and check the output to see that there's consistency in spacing between each section and each important thing within it. We'll want to see that in the testing.

Indentation here is wrong. We do our {% blocktag %} indentation inside the tags themselves and do not increase the indentation of nest tags.
This should be formatted as:

<div style="margin-top: 1.5em">
{%   if review_request.repository %}
 <strong style="....">Repository: </strong>
 {{review_request.repository.name}}
{%   endif %}
{%   if review_request.branch %}
 <strong style="...">Branch: </strong>
 {{review_request.branch}}
{%   endif %}
</div>

It's also not quite right here. Probably need to include a <br>.

This doesn't look right. What we want is:

Repository: Review Board
Branch: release-3.0.x

Ship It!