codeintel: Move gitserver.Head and CommitExists
Created by: ryanslade
Move CommitExists and Head out of the codeintel client and into the git package.
Part of https://github.com/sourcegraph/sourcegraph/issues/27909
Created by: ryanslade
Move CommitExists and Head out of the codeintel client and into the git package.
Part of https://github.com/sourcegraph/sourcegraph/issues/27909