t5801: "VAR=VAL shell_func args" is forbidden

It is not a portable expectation that a single-shot environment
variable assignment works when calling a shell function, not a
command.

Set and export the variable before calling "test_must_fail git push"
instead.  This change would not hurt because this is the last
command in the subprocess and the environment will not seep through
to later tests without using a single-shot assignment.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t5801-remote-helpers.sh

@@ -118,7 +118,9 @@ test_expect_success 'pushing without refspecs' '
 	(cd local2 &&
 	echo content >>file &&
 	git commit -a -m ten &&
-	GIT_REMOTE_TESTGIT_REFSPEC="" test_must_fail git push 2>../error) &&
+	GIT_REMOTE_TESTGIT_REFSPEC="" &&
+	export GIT_REMOTE_TESTGIT_REFSPEC &&
+	test_must_fail git push 2>../error) &&
 	grep "remote-helper doesn.t support push; refspec needed" error
 '