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)
adding foo
adding bar
adding baz/bletch
test-archive-TIP/bar
test-archive-TIP/baz/bletch
test-archive-TIP/foo
test-archive-TIP/bar
test-archive-TIP/baz/bletch
test-archive-TIP/foo
Archive: archive.zip
testing: test-archive-TIP/bar OK
testing: test-archive-TIP/baz/bletch OK
testing: test-archive-TIP/foo OK
No errors detected in compressed data of archive.zip.