t4200: convert sed expression which operates on non-text file to perl

POSIX only requires sed to work on text files and MERGE_RR is not a text
file.  Some versions of sed complain that this file is not newline
terminated, and exit non-zero.  Use perl instead which does not have a
problem with it.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4200-rerere.sh

@@ -57,7 +57,7 @@ test_expect_success 'conflicting merge' '
 	test_must_fail git merge first
 '
 
-sha1=$(sed -e 's/	.*//' .git/MERGE_RR)
+sha1=$(perl -pe 's/	.*//' .git/MERGE_RR)
 rr=.git/rr-cache/$sha1
 test_expect_success 'recorded preimage' "grep ^=======$ $rr/preimage"