Prompt to use old .git/GIT_HUB_EDIT_MSG

[ingydotnet]

When starting an editor and old file exists, prompt to use it. New file will
overwrite old one, so no need to delete.

[ingydotnet]

We do need to delete the file on success though, so we know not to reuse it.

Also delete it if no msg was entered.

Also might only want to reuse it if less than 10 mins old.