t3060: fix mention of function prune_index

Commit [1] added tests which trigger function prune_cache.  The comments
in these tests, however, incorrectly call it "prune_path".  Since then,
function "prune_cache" has been renamed to "prune_index" in commit [2].
Later still in commit [3], the_index singleton, which is also mentioned
in a comment, stopped being used directly with function "prune_index".

Fix mentions of function "prune_index" and the struct it changes in
comments in file "t3060-ls-files-with-tree.sh".

[1] 54e1abce90 (Add test case for ls-files --with-tree, 2007-10-03)
[2] 6510ae173a (ls-files: convert prune_cache to take an index,
    2017-06-12)
[3] 188dce131f (ls-files: use repository object, 2017-06-22)

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3060-ls-files-with-tree.sh

@@ -40,7 +40,7 @@ test_expect_success 'setup' '
 	git commit -a -m "remove them all" &&
 
 	# The bug also requires some entry before our directory so that
-	# prune_path will modify the_index.cache
+	# prune_index will modify the_repository->index.cache
 
 	mkdir a_directory_that_sorts_before_sub &&
 	>a_directory_that_sorts_before_sub/file &&
@@ -56,7 +56,7 @@ test_expect_success 'usage' '
 '
 
 test_expect_success 'git ls-files --with-tree should succeed from subdir' '
-	# We have to run from a sub-directory to trigger prune_path
+	# We have to run from a sub-directory to trigger prune_index
 	# Then we finally get to run our --with-tree test
 	(
 		cd sub &&