tests: consolidate CR removal/addition functions

append_cr(), remove_cr(), q_to_nul() and q_to_cr() are defined in multiple
tests. Consolidate them into test-lib.sh so we can stop redefining them.
The use of remove_cr() in t0020 to test for a CR is replaced with a new
function has_cr() to accurately reflect what is intended (the output of
remove_cr() was being thrown away).

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4012-diff-binary.sh

@@ -77,10 +77,6 @@ test_expect_success 'apply binary patch' \
 	 tree1=`git write-tree` &&
 	 test "$tree1" = "$tree0"'
 
-q_to_nul() {
-	perl -pe 'y/Q/\000/'
-}
-
 nul_to_q() {
 	perl -pe 'y/\000/Q/'
 }