repo: Do not use search path to find refs/remotes/origin/<branch>

repo: Do not use search path to find refs/remotes/origin/<branch>

When running from a non-bare repository, "jgit repo" checks whether
the rev passed in is a sha1 or branch name and in the latter case will
check out origin/<branch>.

We are expecting refs/remotes/origin/<branch>, but as a side effect of
using getRef we also end up looking for refs/origin/<branch>,
refs/heads/origin/<branch>, and so on.  Avoid that by using exactRef
instead.

Signed-off-by: Jonathan Nieder <jrn@google.com>
Change-Id: I670b2e48a88138a1f2104ea201baa958e9edbddb