[IMP] not overwrite migration issue line unless the previous PR is closed

[legalsylvain]

See : #191 (comment)

@pedrobaeza Said :
"my initial intention was to always overwrite, thinking that maintainers will know what to do, but now seeing in perspective, it seems that people don't look too much, so that's why I say to change the initial intention to prevent such change unless the PR is closed"

[sbidoul]

@legalsylvain can we close this one?

[legalsylvain]

No. #191 (comment) didn't changed the bot feature regarding this topic.
Still todo.