Bump gitaly client
Bump gitaly client to 51da8bc17059e4ccd39873e4f3def935341472b8 to get changes from gitaly!4439 (merged).
Also see gitaly#4132 (closed) and gitlab-com/gl-infra&603 (comment 884415245).
This should reduce the gitlab-sshd heap size when many sidechannels are open.
Edited by Jacob Vosmaer