executors: Forward git to gitserver directly

Created by: eseliger

The current implementation does the following:

• Take the request
• Forward it to the internal api on the same machine
• That then redirects to gitserver
• And our reverse proxy follows that redirect
• And finally returns the result to the user

We can save one round trip by simply talking to gitserver directly.

Test plan

Verified locally it still works and adjusted test suites.