diff-tree: read the index so attribute checks work in bare repositories

A regression was introduced in 557a5998d (submodule: remove
gitmodules_config, 2017-08-03) to how attribute processing was handled
in bare repositories when running the diff-tree command.

By default the attribute system will first try to read ".gitattribute"
files from the working tree and then falls back to reading them from the
index if there isn't a copy checked out in the worktree.  Prior to
557a5998d the index was read as a side effect of the call to
'gitmodules_config()' which ensured that the index was already populated
before entering the attribute subsystem.

Since the call to 'gitmodules_config()' was removed the index is no
longer being read so when the attribute system tries to read from the
in-memory index it doesn't find any ".gitattribute" entries effectively
ignoring any configured attributes.

Fix this by explicitly reading the index during the setup of diff-tree.

Reported-by: Ben Boeckel <ben.boeckel@kitware.com>
Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-tree.c

@@ -110,6 +110,8 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
 
 	git_config(git_diff_basic_config, NULL); /* no "diff" UI options */
 	init_revisions(opt, prefix);
+	if (read_cache() < 0)
+		die(_("index file corrupt"));
 	opt->abbrev = 0;
 	opt->diff = 1;
 	opt->disable_stdin = 1;

t/t4015-diff-whitespace.sh

@@ -608,6 +608,23 @@ test_expect_success 'check with space before tab in indent (diff-tree)' '
 	test_must_fail git diff-tree --check HEAD^ HEAD
 '
 
+test_expect_success 'check with ignored trailing whitespace attr (diff-tree)' '
+	test_when_finished "git reset --hard HEAD^" &&
+
+	# create a whitespace error that should be ignored
+	echo "* -whitespace" >.gitattributes &&
+	git add .gitattributes &&
+	echo "foo(); " >x &&
+	git add x &&
+	git commit -m "add trailing space" &&
+
+	# with a worktree diff-tree ignores the whitespace error
+	git diff-tree --root --check HEAD &&
+
+	# without a worktree diff-tree still ignores the whitespace error
+	git -C .git diff-tree --root --check HEAD
+'
+
 test_expect_success 'check trailing whitespace (trailing-space: off)' '
 	git config core.whitespace "-trailing-space" &&
 	echo "foo ();   " >x &&