t1004: insert missing "branch" in a message

The message in question reads awkward with the name "master", but will
be even more confusing once that is renamed to "main". Let's adjust it
in advance of said rename.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1004-read-tree-m-u-wf.sh

@@ -20,7 +20,7 @@ test_expect_success 'two-way setup' '
 	git branch side &&
 	git tag -f branch-point &&
 
-	echo file2 is not tracked on the master anymore &&
+	echo file2 is not tracked on the master branch anymore &&
 	rm -f file2 subdir/file2 &&
 	git update-index --remove file2 subdir/file2 &&
 	git commit -a -m "master removes file2 and subdir/file2"