First line of Returns needs to be tuple:

We can probably get rid of this test. There's no logic besides a fixed return value in get_sort_field, and it won't change unless we want it to, in which case this test also has to be changed. Tests that test something that can be impacted by other changes are more valuable.

No period at the end of unit test docstrings.

""" should be on the previous line.
Also, no period at the end of unit test docstrings.

A few things of note here:

The context object here is technically a ContextList object, lets you access individual keys without worrying about which sub-context it's in. So when we know what the exact key is that we want to look up, we can just grab it.
It's not really safe or efficient to loop through all items in a context. We can get way too many false-positives through that. So that ties into #1 as well.
In general, it's better to collapse nested if statements like this, but we probably don't need that first one if we follow #1 and #2.

What object are those if statements meant to compare with? If we pull that out and check it, this will be a much simpler, faster, safer test.

Same docstring comments apply here.

The reviewboard/datagrids/tests.py file runs all tests successfully, when using the associated djblets patch.

Since we're making this column sortable as well, there should be a unit test testing this as well. That way, you know that it's sorting the way you expect it to.

You do not need Review Board: in your summary since it will land in RB (and shows repository: Review Board in details).

Your code only affects the FullNameColumn, so I would suggest slightly re-wording the summary to make that clear, e.g.



Make sotrable the full name column on user  datagrids

This is because another column may be added in the future that may not be sortable and could lead to a confusing git blame.

A 2-tuple containing the fields to use to sort a user by their full name.

I think the first sentence of your description is a bit confusing (I had to read it a couple times to make sense of it, I think because you split up the sentence with the names of the three columns so by the time I get to "sortable" I forgot what the subject of the sentence was).
You could simplify this to be "This fix makes all three columns of the /users/ page sortable by first and last name (the columns include UsernameColumn, FullNameColumn and PendingCountColumn)."

It might be more clear to simply say Retrieve the first name and last name fields.

It might be more clear to simply say Tuple containing the first name and last name fields.

It might be clearer to say "Testing sorting of FullNameColumn in ascending order"