Reduce amount of calls to git config

[eseliger]

We have a lot of mechanisms revolving around git config, to store sourcegraph internal state.

This is by far the most frequently run exec call on sourcegraph.com so we should investigate how we can cause less load from spawning all these millions of processes in a 3h window.

https://ui.honeycomb.io/sourcegraph/board/DrqVyyy4nWq/Gitserver-DotCom?endTime=0&startTime=-1800

[eseliger]

Made some progress here: #62615

The number of processes spawned for git config went from ~110 to ~96 on S2, and from 1150 to 750 on dotcom.

Still lots of room for improvement.