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>
diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index 98d1cbe..733f626 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -634,6 +634,8 @@
 			if (!strcmp(arg, "--show-prefix")) {
 				if (prefix)
 					puts(prefix);
+				else
+					putchar('\n');
 				continue;
 			}
 			if (!strcmp(arg, "--show-cdup")) {