t4011: illustrate "diff-index -p" on stat-dirty paths

The plumbing that looks at the working tree, i.e. "diff-index" and
"diff-files", always emit the "diff --git a/path b/path" header lines
without anything else for paths that are only stat-dirty (i.e. different
only because the cached stat information in the index no longer matches
that of the working tree, but the real contents are the same), when
these commands are run with "-p" option to produce patches.

Illustrate this current behaviour.  Also demonstrate that with the "-w"
option, we (correctly) hold off showing a "diff --git" header until actual
differences have been found.  This also suppresses the header for merely
stat-dirty files, which is inconsistent.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t4011-diff-symlink.sh

@@ -9,7 +9,7 @@ test_description='Test diff of symlinks.
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/diff-lib.sh
 
-test_expect_success SYMLINKS 'diff new symlink' '
+test_expect_success SYMLINKS 'diff new symlink and file' '
 	cat >expected <<-\EOF &&
 	diff --git a/frotz b/frotz
 	new file mode 120000
@@ -19,22 +19,30 @@ test_expect_success SYMLINKS 'diff new symlink' '
 	@@ -0,0 +1 @@
 	+xyzzy
 	\ No newline at end of file
+	diff --git a/nitfol b/nitfol
+	new file mode 100644
+	index 0000000..7c465af
+	--- /dev/null
+	+++ b/nitfol
+	@@ -0,0 +1 @@
+	+xyzzy
 	EOF
 	ln -s xyzzy frotz &&
+	echo xyzzy >nitfol &&
 	git update-index &&
 	tree=$(git write-tree) &&
-	git update-index --add frotz &&
+	git update-index --add frotz nitfol &&
 	GIT_DIFF_OPTS=--unified=0 git diff-index -M -p $tree >current &&
 	compare_diff_patch expected current
 '
 
-test_expect_success SYMLINKS 'diff unchanged symlink'  '
+test_expect_success SYMLINKS 'diff unchanged symlink and file'  '
 	tree=$(git write-tree) &&
-	git update-index frotz &&
+	git update-index frotz nitfol &&
 	test -z "$(git diff-index --name-only $tree)"
 '
 
-test_expect_success SYMLINKS 'diff removed symlink' '
+test_expect_success SYMLINKS 'diff removed symlink and file' '
 	cat >expected <<-\EOF &&
 	diff --git a/frotz b/frotz
 	deleted file mode 120000
@@ -44,22 +52,38 @@ test_expect_success SYMLINKS 'diff removed symlink' '
 	@@ -1 +0,0 @@
 	-xyzzy
 	\ No newline at end of file
+	diff --git a/nitfol b/nitfol
+	deleted file mode 100644
+	index 7c465af..0000000
+	--- a/nitfol
+	+++ /dev/null
+	@@ -1 +0,0 @@
+	-xyzzy
 	EOF
 	mv frotz frotz2 &&
+	mv nitfol nitfol2 &&
 	git diff-index -M -p $tree >current &&
 	compare_diff_patch expected current
 '
 
-test_expect_success SYMLINKS 'diff identical, but newly created symlink' '
+test_expect_success SYMLINKS 'diff identical, but newly created symlink and file' '
 	cat >expected <<-\EOF &&
 	diff --git a/frotz b/frotz
+	diff --git a/nitfol b/nitfol
 	EOF
+	rm -f frotz nitfol &&
+	echo xyzzy >nitfol &&
+	test-chmtime +10 nitfol &&
 	ln -s xyzzy frotz &&
 	git diff-index -M -p $tree >current &&
+	compare_diff_patch expected current &&
+
+	>expected &&
+	git diff-index -M -p -w $tree >current &&
 	compare_diff_patch expected current
 '
 
-test_expect_success SYMLINKS 'diff different symlink' '
+test_expect_success SYMLINKS 'diff different symlink and file' '
 	cat >expected <<-\EOF &&
 	diff --git a/frotz b/frotz
 	index 7c465af..df1db54 120000
@@ -70,9 +94,17 @@ test_expect_success SYMLINKS 'diff different symlink' '
 	\ No newline at end of file
 	+yxyyz
 	\ No newline at end of file
+	diff --git a/nitfol b/nitfol
+	index 7c465af..df1db54 100644
+	--- a/nitfol
+	+++ b/nitfol
+	@@ -1 +1 @@
+	-xyzzy
+	+yxyyz
 	EOF
 	rm -f frotz &&
 	ln -s yxyyz frotz &&
+	echo yxyyz >nitfol &&
 	git diff-index -M -p $tree >current &&
 	compare_diff_patch expected current
 '