completions: Fix bash completions when a option requires a argument
completions: Fix bash completions when a option requires a argument

Since the string of options variable as pattern in the case statement has
not been delimited and it does not match the value of prev variable,
bash completions tries to complement any option even when a specified
option requires a argument.
This fix stops complementing options when a option requires a argument.

Signed-off-by: ERAMOTO Masaya <495370e91cae577581028a5efee5824f74357ca0@jp.fujitsu.com>

mtrmac/skopeo,projectatomic/skopeo,mtrmac/skopeo,mtrmac/skopeo,projectatomic/skopeo