merge: Avoid overwriting the output id column with non-id columns

[tsibley]

This edge case occurs when an input table has an id column used for joining (in tests, literally "id", but any valid id column name) and also another, non-id column that matches the output id column name (in tests, literally "strain").

Checklist

• [x] Automated checks pass
• [x] Check if you need to add a changelog message
• [x] Check if you need to add tests
• [x] Check if you need to update docs

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.03%. Comparing base (d662698) to head (232698b). Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1593 +/- ## ======================================= Coverage 71.02% 71.03% ======================================= Files 79 79 Lines 8256 8258 +2 Branches 2003 2004 +1 ======================================= + Hits 5864 5866 +2 Misses 2101 2101 Partials 291 291 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[tsibley]

The linkcheck step in CI has now failed twice in different ways: first reporting a 403 Forbidden for http://www.microbesonline.org/fasttree/ and then reporting 406 Unacceptable for https://github.com/nextstrain/augur/commit/4859b5d70e77cc9a0bb99e741fefb29952058b71. Third time was the charm. The only thing I did was re-run the job.