We've improved the speed of /submitted_together inside Google recently
and found two bottlenecks on the API layer resulting from opening every
repo that we return changes for. For large topics like the one Android
uses, this can take >1 minute.
Change I763d1544 removed a default option that required lazy loading in
ChangeJson. This commit tweaks the last bit: WalkSorter which opens
all of the repos to perform a topo-sort.
We don't guarantee any sorting in the API docs for /submitted_together
and sorting by project and numeric ID (hence change creation time)
sounds like a reasonable and *much* faster alternative.
This commit keeps WalkSorter for cases where we can return a result
reasonably fast and uses other sorting techniques where applicable.
Change-Id: I1155b936c19d31b1b1b58ff51f9e73aa4445779d
fe8727e366