t3010: update to demonstrate "ls-files -k" optimization pitfalls

An earlier draft of the previous step used cache_name_exists() to
check the directory we were looking at, which missed the second case
described in its log message.  Demonstrate why it is not sufficient.

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

t/t3010-ls-files-killed-modified.sh

@@ -11,6 +11,7 @@ This test prepares the following in the cache:
     path1       - a symlink
     path2/file2 - a file in a directory
     path3/file3 - a file in a directory
+    pathx/ju    - a file in a directory
 
 and the following on the filesystem:
 
@@ -21,6 +22,7 @@ and the following on the filesystem:
     path4	- a file
     path5	- a symlink
     path6/file6 - a file in a directory
+    pathx/ju/nk - a file in a directory to be killed
 
 git ls-files -k should report that existing filesystem
 objects except path4, path5 and path6/file6 to be killed.
@@ -44,16 +46,17 @@ then
 else
 	date > path1
 fi
-mkdir path2 path3
+mkdir path2 path3 pathx
 date >path2/file2
 date >path3/file3
+>pathx/ju
 : >path7
 date >path8
 : >path9
 date >path10
 test_expect_success \
     'git update-index --add to add various paths.' \
-    "git update-index --add -- path0 path1 path?/file? path7 path8 path9 path10"
+    "git update-index --add -- path0 path1 path?/file? pathx/ju path7 path8 path9 path10"
 
 rm -fr path? ;# leave path10 alone
 date >path2
@@ -65,14 +68,15 @@ else
 	date > path3
 	date > path5
 fi
-mkdir path0 path1 path6
+mkdir -p path0 path1 path6 pathx/ju
 date >path0/file0
 date >path1/file1
 date >path6/file6
 date >path7
 : >path8
 : >path9
 touch path10
+>pathx/ju/nk
 
 test_expect_success \
     'git ls-files -k to show killed files.' \
@@ -82,6 +86,7 @@ path0/file0
 path1/file1
 path2
 path3
+pathx/ju/nk
 EOF
 
 test_expect_success \
@@ -98,6 +103,7 @@ path2/file2
 path3/file3
 path7
 path8
+pathx/ju
 EOF
 
 test_expect_success \