This allows automation to prefill the subject/body for the editor.
It could also be useful if some kind of "amend commit" functionality
was implemented.
Closes: #305
Approved by: cgwalters
Previously, the behavior was to run the editor if no subject or body was specified.
This led to using commit -s '' in scripts, to avoid running the editor.
In practice, manually editing commit messages is rare, whereas automated scripting
is common, so instead default to not running the editor and only run it when -e
is given.
Closes: #305
Approved by: cgwalters