ls-tree $di $dir: do not mistakenly recurse into directories

When applying two pathspecs, one of which is named as a prefix to the
other, we mistakenly recursed into the shorter one.

Noticed and fixed by David Reis.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3100-ls-tree-restrict.sh

@@ -165,4 +165,13 @@ test_expect_success \
 EOF
      test_output'
 
+test_expect_success \
+    'ls-tree with one path a prefix of the other' \
+    'git ls-tree $tree path2/baz path2/bazbo >current &&
+     make_expected <<\EOF &&
+040000 tree X	path2/baz
+120000 blob X	path2/bazbo
+EOF
+     test_output'
+
 test_done