commandstatshandler: Fix flaky test caused by Git version check

In our commandstatshandler we're asserting that when executing RPCs, we record the number of spawned commands. To do so we use an RPC that spawns a Git command. Due to recent changes in our Git command factory we had to adapt the test though as we are now additionaly spawning a git-version(1) command to auto-detect the version.

This change is causing flakiness though because we cache the Git version per binary that is executed: we don't reuse the context, and when running with bundled Git we now thus randomly use either the old or the new bundled Git binaries based on a feature flag that uses a random value. So based on whether this value is the same across both RPC calls or not we may or may not re-execute git-version(1).

Fix this by reusing the same context for both RPC calls to guarantee that the feature flag has the same state.

Fixes #4239 (closed)