Move git.ListRefs and git.ListBranches onto gitserver client
Created by: ryanslade
This moves the last non test code out of the vcs/git
package. Another PR will move tests into
the gitserver package and remove vcs/git
.
Closes https://github.com/sourcegraph/sourcegraph/issues/37561 Closes https://github.com/sourcegraph/sourcegraph/issues/36535
Test plan
Existing tests still pass