Quote arguments to tr in test-lib

When there are single-character filenames in the test directory,
the shell tries to expand regexps meant for tr.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

t/test-lib.sh

@@ -34,7 +34,7 @@ export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
 export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export EDITOR VISUAL
 
-case $(echo $GIT_TRACE |tr [A-Z] [a-z]) in
+case $(echo $GIT_TRACE |tr "[A-Z]" "[a-z]") in
 	1|2|true)
 		echo "* warning: Some tests will not work if GIT_TRACE" \
 			"is set as to trace on STDERR ! *"