When something went wrong and git launches editor, and a editor chosen
was terminal-based, a test run unnoticeably hangs.

As we intend editors not to be ran with --no-edit, GIT_EDITOR should be
defaulted to `false` so let Git command fails when it attempts to start a
editor. This allows catching such unintentional behaviour in test suite.

(Note: Prior to Git v2.32.0, git cherry-pick --no-edit doesn't work for
conflict resolution so currently the test hangs when older Git is in use.
https://github.com/git/git/commit/39edfd5cbc4d168db19ec1bc867d78ec7211ec39
i.e. Ubuntu 20.04, Debian Bullseye doesn't satisfy this prerequisite.)