code-review-start: Add `get_merge_base` function

Function to get the merge base for the current branch. If none exists in
the database, default to `origin/master` if there's an `origin` remote,
or `master` if not.

Thanks to Serpiton
(https://codereview.stackexchange.com/users/61525/serpiton) on Stack
Overflow for describing how to get a default value using the `MAX()`
function:
https://codereview.stackexchange.com/questions/74192/select-first-value-if-exists-otherwise-select-default-value/74252#74252

Thanks to 'tig' (https://stackoverflow.com/users/96823/tig) on Stack
Overflow for the command to check if a given Git remote exists:
https://stackoverflow.com/questions/12170459/check-if-git-remote-exists-before-first-push/26843249#26843249

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