Ryan McElroy <rmcelroy@fb.com> [Tue, 10 Nov 2015 11:13:21 -0800] rev 26935
exchange: pass pushop to discovery.checkheads
Previously, we passed a bunch of parameters to discovery.checkheads, but all
of the arguments can be fetched out of pushop, which may contain a lot more
useful information for extensions now that pushop is extensible.