prevent exit code 1 when deleting a snapshot in quiet mode (-q)

[lpirl]

When in quiet mode (i.e., -q is given) false negative exit codes might occur.
For example, when snapshots have to be deleted due to rotation.
This is especially annoying with (various) cron (implementations), because non-zero exit codes trigger an email being sent although no actual error occurred.
The cause might be the line test -z "$quiet" && echo "$1" in function log.info() (e.g., in 1.7.3:

btrfs-snap/btrfs-snap

Lines 183 to 186 in 7126515

	function log.info() {
	logger -p ${LOG_FACILITY}.info -t ${prog} "$1"
	test -z "$quiet" && echo "$1"
	}

) which does not "mask" negative exit codes of test.

[lpirl]

Since this repository does not seem to be maintained by @jf647 anymore, I meanwhile merged this into https://github.com/lpirl/btrfs-snap/tree/master .