Fix bug in dagutils MergeDiffs.

[nasdf]

This fixes a bug where conflicts were also returned as changes when merged.

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[rvagg]

I think this is OK, but would you mind trying to add a simple test case for it?

[nasdf]

Yeah I'll update when I have some free time this week.

[nasdf]

@rvagg I cleaned up the diff function and added some tests for it as well. Hopefully this all looks good and can be merged soon.

[nasdf]

@Stebalien Can you take a look at this?

[aschmahmann]

@nasdf a lot of people are unavailable over the next little bit due to holidays (Dec - Jan is frequently rough for getting things merged in). Things should start picking back up later this month and then we can hopefully get this taken care of.

[rvagg]

This is a very nice code cleanup in general, and tests, and I think it's just the single delete that might be doing all of the fix work here, so thanks for going the extra mile! lgtm.