Add an explicit "null" to a missed diffusion.branchquery callsite to fix Diffusion "Branches" page

Summary:
See PHI775. See D19499. Originally, see PHI720.

D19499 broke the standalone "Branches" page for commits. Normally, you reach this by taking these steps:

• View a commit which is contained by 11 or more branches.
• Click the "More Branches..." link in the "Branches" field.
• You should be taken to a list of all branches which contain the commit.

The change to the 'branch' parameter was adjusted in the query that builds the "x, y, z, More Branches..." list, but not on the actual "Branches" list with the full list. Adjust it.

Test Plan:

• Set display limit to 1, viewed a commit on "master" and "stable", clicked "More Branches".
• Before: saw only "master".
• After: saw both "master" and "stable".

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19532