config: fail if --get-urlmatch finds no value

config: fail if --get-urlmatch finds no value

The --get, --get-all and --get-regexp options to git-config exit with
status 1 if the key is not found but --get-urlmatch succeeds in this
case.

Change --get-urlmatch to behave in the same way as the other --get*
options so that all four are consistent.  --get-color is a special case
because it accepts a default value to return and so should not return an
error if the key is not found.

Also clarify this behaviour in the documentation.

Signed-off-by: John Keeping <redacted>
Signed-off-by: Junio C Hamano <redacted>