tests: Set EDITOR=: and VISUAL=: globally

This way we don't have to remember to set it for each test; and
if we forget, we won't cause interactive editors to be spawned
for non-interactive tests.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t4013-diff-various.sh

@@ -70,7 +70,7 @@ test_expect_success setup '
 	for i in 1 2; do echo $i; done >>dir/sub &&
 	git update-index file0 dir/sub &&
 
-	EDITOR=: VISUAL=: git commit --amend &&
+	git commit --amend &&
 	git show-branch
 '