Compute GitMaxConcurrentClones and use it everywhere
Created by: indradhanush
Inspiration came from this review thread: https://github.com/sourcegraph/sourcegraph/pull/24892/files/f0c22b71cd40ffaf44030977c840b4c1e47564c5#r708614129
Link to Sourcegraph search results for GitMaxConcurrentClones
in Go
files in this tree to ensure I didn't miss anything: https://sourcegraph.com/search?q=context:global+repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24%404a4e075+GitMaxConcurrentClones+lang:Go+&patternType=literal&case=yes