BUG-1590: introduce PathArgument.toRelativeString() and use it

This patch introduces a new method, toRelativeString(), which does the
same thing as toString(), but takes into account previous path argument
and does not emit namespace if it matches the previous namespace.

Change-Id: Ia75c7cbf4a2fd7639a7c991e5b8242bd5f2a439a
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1590: introduce PathArgument.toRelativeString() and use it"