Since git core.editor was set in the initial commit, I suspect I wasn't
aware of $EDITOR in the environment. So it's safe to delete
git/gitconfig's core.editor and take it from $EDITOR by default, which
actually checks if nvim is installed.
for git's diff.tool and merge.tool, I don't see a possibility to have a
conditional switch and fallback to vim. I don't expect to work on
another workstation without neovim.
Convert all shebangs to /usr/bin/env bash, if bash is really required in
the script. Otherwise convert it to /bin/sh.
/bin/bash is not available on all systems, while /usr/bin/env bash and
/bin/sh are.
