Rebase creates conflict if both "our" and "their" changeset do the same change

[wonder-sk]

As the title says - if two users do an update of a feature independently, but the actual change is the same (e.g. both change an attribute value of the same feature from 40 to 42), geodiff will generate a rebase conflict file. Ideally this should not happen, because there wasn't really any conflict, so we should detect such cases and skip reporting conflicts, because that's confusing to users if they see conflict files appearing.

Try to run this command using test data from geodiff:

$ geodiff rebase-diff tests/testdata/base.gpkg \
    tests/testdata/1_geopackage/base-modified_1_geom.diff \
    tests/testdata/1_geopackage/base-modified_1_geom.diff \
    /tmp/x_rebased.diff /tmp/x_conflict.json

In short, we are trying to rebase a diff on top of the same diff, so there should be no conflict and the rebased diff should be empty. But instead we are getting a conflict file:

{
   "geodiff": [
      {
        "table": "gpkg_contents",
        "type": "conflict",
        "fid": "334167370",
        "changes": [
          {
              "column": 5,
              "base": -1.0889200000000001,
              "old": -0.73050599999999999,
              "new": -0.73050599999999999
          }
        ]
      },
      {
        "table": "simple",
        "type": "conflict",
        "fid": "1",
        "changes": [
          {
              "column": 1,
              "base": "R1AAAeYQAAABAQAAAB54y6E2bPG/cOaqyDmB3T8=",
              "old": "R1AAAeYQAAABAQAAAClvCOFAFeG/lhl3A7Qi0z8=",
              "new": "R1AAAeYQAAABAQAAAClvCOFAFeG/lhl3A7Qi0z8="
          }
        ]
      }
   ]
}

(the gpkg_contents changes should have been ignored when reading the source diff, that's probably another minor issue to handle)

And the rebased diff looks like this instead of being empty:

{
   "geodiff": [
      {
        "table": "gpkg_contents",
        "type": "update",
        "changes": [
          {
              "column": 0,
              "old": "simple"
          },
          {
              "column": 4,
              "old": "2019-06-26T08:19:00.735Z",
              "new": "2019-06-26T08:19:00.735Z"
          },
          {
              "column": 5,
              "old": -0.73050599999999999,
              "new": -0.73050599999999999
          }
        ]
      },
      {
        "table": "simple",
        "type": "update",
        "changes": [
          {
              "column": 0,
              "old": 1
          },
          {
              "column": 1,
              "old": "R1AAAeYQAAABAQAAAClvCOFAFeG/lhl3A7Qi0z8=",
              "new": "R1AAAeYQAAABAQAAAClvCOFAFeG/lhl3A7Qi0z8="
          }
        ]
      }
   ]
}