Replace in-place sed in t7502-commit

The in-place mode of sed used in t7502-commit is a non-POSIX extension.
That call of sed is replaced by a more portable version using a temporary file.

Signed-off-by: Marcel Koeppen <git-dev@marzelpan.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7502-commit.sh

@@ -166,7 +166,9 @@ test_expect_success 'author different from committer' '
 	test_cmp expect actual
 '
 
-sed -i '$d' expect
+mv expect expect.tmp
+sed '$d' < expect.tmp > expect
+rm -f expect.tmp
 echo "# Committer:
 #" >> expect
 unset GIT_COMMITTER_EMAIL