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.)

After editing a conflict, continue without removed files.

The case of 7fc73ab5f6fbe46655855079954b26dcc14576b3, which modified
`.gitignore` and `.github/workflows/main.yml`.  Both files need to be
rejected and restored, but since the latter file was not there before,
`git checkout` failed and the former file could not be restored along
with it.  To fix this failure, restore the ignored files one by one.

When multiple files changed in the same directory to be removed, the
first file only was removed from changed set.

Referencing always after conversion to a symbol, and yielded gem name
is always converted to a string.

Do not add directories which would let chkbuild fires accidentally, to
just under `test`.  The "fixtures" directory is often used to place
test stuffs.