I'm unsure about this change. On one hand, the comment (which appears to have been added to mpm several years ago) implies that we never should have generated backups in this case. On the other, one of the key rules of a VCS is "don't eat my data." Even though we are aborting the operation, I could see some scenarios where someone would want a backup of the aborted/partially-completed rebase.
@durin42: what do you think?