Don't abort when backup files already exist. Don't allow alternative names.
Rationale:
- When the user wants to revert, he shouldn't be stopped from doing
this just because some old backups will be overwritten.
- To not clobber important files by accident, alternative names for backup
files were disabled. As the backup target now has a fixed name, the user
doesn't have to be informed about the backup copy (unless --verbose)
abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'