Support template files that only included partials

[mschoettle]

I have a template file that includes a partial:

{% load i18n %}

{% partialdef some-partial %}
  ...
{% endpartialdef %}

And templates that include the partial as follows:

{% include "app/_partials.html#some-partial" %}

And I have tests that request URLs of views that use a template that include the partial. However, the reported coverage states lines 1-3 as missing:

project/templates/app_partials.html       6      3      0      0  50.00%   1-3