Treat CloneCommand.setRemote(null) as setRemote("origin")

Treat CloneCommand.setRemote(null) as setRemote("origin")

A non-bare clone command with null remote produces a
NullPointerException when trying to produce a refspec to fetch against.

In a bare repository, a null remote name is accepted by mistake,
producing a configuration with items like 'remote.url' instead of
'remote.<remote>.url'.  This was never meant to work.

Instead, let's make setRemote(null) undo any previous setRemote calls
and re-set the remote name to DEFAULT_REMOTE, imitating C git clone's
--no-origin option.

While we're here, add some tests for setRemote working normally.

Change-Id: I76f502da5e677df501d3ef387e7f61f42a7ca238
Signed-off-by: Jonathan Nieder <jrn@google.com>