Add a helper function to DiffusionPathIDQuery

Summary:
Just breaking D960 into some smaller parts, this is a standalone method used in
Mercurial parsing.

(There's a bad version of this function in the SVN stuff but I'll get rid of it
the next time I'm in there.)

Test Plan: See D960.

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

Reviewed By: Makinde

CC: aran, Makinde

Differential Revision: 965