graphql: Record queries that trigger getCommit calls

Created by: tsenart

This is an attempt to debug #15392 (closed) and find what GraphQL queries trigger the getCommit codepath, which leads to a spike of git log calls in git server, that triggers that alert.

In the future, we should invest in working tracing, rather than only per-request activated tracing, which doesn't allow us to retroactively debug situations like these.