Diff command output has strange order if input files don't have equal set of metadata attributes set

[Nakaner]

This is related to #90. The same command and input was used. The output looks like this:

+n10 v1 dV c0 t i0 u T x1 y1
-n10 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T x1 y1
-n11 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T x1 y2
+n11 v2 dV c0 t i0 u T x2 y2
+n12 v1 dV c0 t i0 u T x1 y3
-n12 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T x1 y3
-n13 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T x1 y4
+n14 v1 dV c0 t i0 u T x1 y5
+n15 v1 dV c0 t i0 u T x1 y5
-n15 v1 dV c1 t2015-01-01T02:00:00Z i2 utest T x1 y5
+n16 v1 dV c0 t i0 u T x1 y5
-n16 v1 dV c1 t2015-01-01T02:00:00Z i1 utest T x2 y5
+w20 v1 dV c0 t i0 u Tfoo=bar Nn10,n11,n12
-w20 v1 dV c1 t2015-01-01T01:00:00Z i1 utest Tfoo=bar Nn10,n11,n12
-w21 v1 dV c1 t2015-01-01T01:00:00Z i1 utest Txyz=abc Nn12,n13
+w21 v2 dV c0 t i0 u Txyz=new Nn12,n14
+r30 v1 dV c0 t i0 u T Mn12@m1,w20@m2
-r30 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T Mn12@m1,w20@m2

I think that +n10 should appear after -n10. The same applies to other lines where only the metadata was different. If I use osmium::object_order_type_id_version_without_timestamp instead of osmium::OSMObject::operator< to compare two objects, the order is correct:

michael@hatano:~/git/osmium-tool$ build/osmium diff --ignore-timestamps --output-format opl test/diff/input1.osm test/diff/input2-only-version.osm 
[======================================================================] 100% 
-n10 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T x1 y1
+n10 v1 dV c0 t i0 u T x1 y1
-n11 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T x1 y2
+n11 v2 dV c0 t i0 u T x2 y2
-n12 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T x1 y3
+n12 v1 dV c0 t i0 u T x1 y3
-n13 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T x1 y4
+n14 v1 dV c0 t i0 u T x1 y5
-n15 v1 dV c1 t2015-01-01T02:00:00Z i2 utest T x1 y5
+n15 v1 dV c0 t i0 u T x1 y5
-n16 v1 dV c1 t2015-01-01T02:00:00Z i1 utest T x2 y5
+n16 v1 dV c0 t i0 u T x1 y5
-w20 v1 dV c1 t2015-01-01T01:00:00Z i1 utest Tfoo=bar Nn10,n11,n12
+w20 v1 dV c0 t i0 u Tfoo=bar Nn10,n11,n12
-w21 v1 dV c1 t2015-01-01T01:00:00Z i1 utest Txyz=abc Nn12,n13
+w21 v2 dV c0 t i0 u Txyz=new Nn12,n14
-r30 v1 dV c1 t2015-01-01T01:00:00Z i1 utest T Mn12@m1,w20@m2
+r30 v1 dV c0 t i0 u T Mn12@m1,w20@m2

Shall I change the default? Or shall I implement it like in #88?

Some commands currently take the timestamp into account when comparing objects (diff, apply-changes, derive-changes) while other commands don't (merge-changes).