conflicts: promote absent sides if it contains only resolved hunks

[06393993]

This helps jj to preserve a change as a conflict if the user only modifies the resolved hunks in conflict files.

For example, we have the following commit graph:

D
|\
B C
|/
A

• A: empty
• B: add "1" to file "a.txt"
• C: add "2" to file "a.txt"
• D: merge from B and C with conflicts.

If the user appends to the conflict file "a.txt" in change D, jj now still recognizes the file "a.txt" in change D as a conflict file.

Fix #7156

Checklist

If applicable:

• [x] I have added/updated tests to cover my changes

N/A:

• [ ] I have updated CHANGELOG.md
• [ ] I have updated the documentation (README.md, docs/, demos/)
• [ ] I have updated the config schema (cli/src/config-schema.json)

[06393993]

Can you take another look at the commit that derived the executable field and the copy id field from the existing sides if not ambiguous? Thanks.

[martinvonz]

@06393993: just a reminder that this is approved and it sounds like there is no more work required on this PR (other than rebasing it)