context cancellation not propagated

to repro, `make server`, then
```
bazel run //example/client:client -- -addr 127.0.0.1:8081 -method get-changed-targets -remote https://github.com/uber/tango.git -base-sha cf3f54d15c652854f31fb552183cfb26ecde1813 -new-base-sha 59932143f27a3e6c230d444f13397f5167db3cef~1  > /tmp/t.json
```
then ctrl+C, the query is still running on server, not cancelled