move GetBehindAhead function to gitserver package
Created by: sashaostrikov
Part of https://github.com/sourcegraph/sourcegraph/issues/33280
Depends on https://github.com/sourcegraph/sourcegraph/pull/36539
Test plan
Existing tests should pass
Created by: sashaostrikov
Part of https://github.com/sourcegraph/sourcegraph/issues/33280
Depends on https://github.com/sourcegraph/sourcegraph/pull/36539
Existing tests should pass