clean: require -f to do damage by default

This makes the clean.requireForce configuration default to true.
Too many people are burned by typing "git clean" by mistake when
they meant to say "make clean".

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t7300-clean.sh b/t/t7300-clean.sh
index eb0847a..8697213 100755
--- a/t/t7300-clean.sh
+++ b/t/t7300-clean.sh
@@ -7,6 +7,8 @@
 
 . ./test-lib.sh
 
+git config clean.requireForce no
+
 test_expect_success 'setup' '
 
 	mkdir -p src &&
@@ -139,6 +141,13 @@
 
 '
 
+test_expect_success 'clean.requireForce defaults to true' '
+
+	git config --unset clean.requireForce &&
+	! git-clean
+
+'
+
 test_expect_success 'clean.requireForce' '
 
 	git config clean.requireForce true &&