+1 for less toil! 

Some thoughts: 

You can use the admin API: https://godoc.org/google.golang.org/api/appengine/v1
 to figure out the currently deployed git hash, and the previously deployed git hash (IIRC this is trickier than it sounds at first). There may be better methods of course but that's what relnotes.go does.

Then call gitiles to get the set of changes between the two hashes (will have to auth using something like what's in client/gerrit.go) - not sure what the API to do that looks like. Hopefully it won't have to talk to Gerrit instead of gitiles.