Simplify push error message when ref already exists

Simplify push error message when ref already exists

If a client attempts to create a branch that already exists on the
remote side, tell them "already exists" rather than repeat lots of
information about the reference. Previously the error looked like:

  ! [remote rejected] tags/1.3.1 -> 1.3.1 (Ref Ref[refs/tags/1.3.1=e3857ee05...] already exists)

Now it will simply say:

  ! [remote rejected] tags/1.3.1 -> 1.3.1 (already exists)

Change-Id: I96fc67ca8b650052de6e662449a3c5bc8bbc010b