Use refName instead of objectId in the fetch REST Api

When requesting a remote fetch, specify the ref-name to be fetched
instead of the SHA1 of the ref.

Fetching by SHA1 has two issues:
- The remote Git server may disallow exposing the fetch by SHA1
- Fetching the SHA1 does not update the local ref

Change-Id: I52fba1166b0d5df3a498af2702328cc7723b9a41