Also replace options and variables named "master" with "main".

.. and fix some WS issues.

This can happen for instance when using homebrew to install an updated
Bash version.

Gets rid of unnecessary forking of various commands from subshells,
preferring to exec from the subshell. Also removes one unnecessary
subshell fork entirely.

If the staging dir is on a different FS (likely if in /tmp) the various
'mv' operations become disk-heavy copy operations. This commit changes
the staging directory location to a temp location in the same directory
as the repo.

git-change-commit has been around for a while outside of version
control.