rev-parse --show-prefix: add in trailing newline

rev-parse --show-prefix: add in trailing newline

Print out a trailing newline when --show-prefix is run with cwd
at the top level of the tree which results in an empty prefix.
Behavior is now like --show-cdup.

Fixes an expected failure in t1501.

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>