diff.c: move read_index() code back to the caller

This code is only needed for diff-tree (since f0c6b2a2fd ([PATCH]
Optimize diff-tree -[CM] --stdin - 2005-05-27)). Let the caller do the
preparation instead and avoid read_index() in diff.c code.

read_index() should be avoided (in addition to the_index) because it
uses get_index_file() underneath to get the path $GIT_DIR/index. This
effectively pulls the_repository in and may become the only reason to
pull a 'struct repository *' in diff.c. Let's keep the dependencies as
few as possible and kick it back to diff-tree.c

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-tree.c

@@ -163,9 +163,11 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
 		int saved_nrl = 0;
 		int saved_dcctc = 0;
 
-		if (opt->diffopt.detect_rename)
-			opt->diffopt.setup |= (DIFF_SETUP_USE_SIZE_CACHE |
-					       DIFF_SETUP_USE_CACHE);
+		if (opt->diffopt.detect_rename) {
+			if (!the_index.cache)
+				read_index(&the_index);
+			opt->diffopt.setup |= DIFF_SETUP_USE_SIZE_CACHE;
+		}
 		while (fgets(line, sizeof(line), stdin)) {
 			struct object_id oid;
 

diff.c

@@ -4414,16 +4414,6 @@ void diff_setup_done(struct diff_options *options)
 
 	if (options->detect_rename && options->rename_limit < 0)
 		options->rename_limit = diff_rename_limit_default;
-	if (options->setup & DIFF_SETUP_USE_CACHE) {
-		if (!active_cache)
-			/* read-cache does not die even when it fails
-			 * so it is safe for us to do this here.  Also
-			 * it does not smudge active_cache or active_nr
-			 * when it fails, so we do not have to worry about
-			 * cleaning it up ourselves either.
-			 */
-			read_cache();
-	}
 	if (hexsz < options->abbrev)
 		options->abbrev = hexsz; /* full */
 

diff.h

@@ -312,7 +312,6 @@ void diff_change(struct diff_options *,
 struct diff_filepair *diff_unmerge(struct diff_options *, const char *path);
 
 #define DIFF_SETUP_REVERSE      	1
-#define DIFF_SETUP_USE_CACHE		2
 #define DIFF_SETUP_USE_SIZE_CACHE	4
 
 /*