t3206: disable parameter substitution in heredoc

In the first heredoc, parameter substitution is not used so prevent it
from happening in the future (perhaps by accident) by escaping the limit
EOF.

The remaining heredocs use parameter substitution so they cannot be
changed.

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

t/t3206-range-diff.sh

@@ -9,7 +9,7 @@ test_description='range-diff tests'
 
 test_expect_success 'setup' '
 	git fast-import <"$TEST_DIRECTORY"/t3206/history.export &&
-	test_oid_cache <<-EOF
+	test_oid_cache <<-\EOF
 	# topic
 	t1 sha1:4de457d
 	t2 sha1:fccce22