Define ListCommits RPC

The ListCommits RPC is a bi-directional stream to be used to open the
stream once, and on demand request commits from Gitaly.

Part of: gitaly#1487
4 jobs for zj-list-commits in 1 minute and 24 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #194769161
build

00:01:23

passed #194769162
check-grpc-proto-clients

00:01:18

passed #194769164
danger-review

00:00:20

passed #194769160
test

00:01:15