code-review-database(create_merge_base): Upsert merge base

Don't duplicate rows if we call `create_merge_base` multiple times.
Doing so would mean we'd get multiple merge base rows for the same head.

Instead, overwrite the existing merge base for a given head using an
upsert.

Thanks a bunch to MarqueIV
(https://stackoverflow.com/users/168179/marqueiv) on Stack Overflow for
demonstrating a way to do an upsert without the UPSERT syntax. This
enables us to do an upsert on SQLite < 3.24.0. Neat trick.
https://stackoverflow.com/questions/15277373/sqlite-upsert-update-or-insert/38463024#38463024

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