tree-diff: rework diff_tree interface to be sha1 based

In the next commit this will allow to reduce intermediate calls, when
recursing into subtrees - at that stage we know only subtree sha1, and
it is natural for tree walker to start from that phase. For now we do

    diff_tree
        show_path
            diff_tree_sha1
                diff_tree
                    ...

and the change will allow to reduce it to

    diff_tree
        show_path
            diff_tree

Also, it will allow to omit allocating strbuf for each subtree, and just
reuse the common strbuf via playing with its len.

The above-mentioned improvements go in the next 2 patches.

The downside is that try_to_follow_renames(), if active, we cause
re-reading of 2 initial trees, which was negligible based on my timings,
and which is outweighed cogently by the upsides.

NOTE To keep with the current interface and semantics, I needed to
rename the function from diff_tree() to diff_tree_sha1(). As
diff_tree_sha1() was already used, and the function we are talking here
is its more low-level helper, let's use convention for prefixing
such helpers with "ll_". So the final renaming is

    diff_tree() -> ll_diff_tree_sha1()
Signed-off-by: NKirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

tree-diff.c

@@ -141,12 +141,17 @@ static void skip_uninteresting(struct tree_desc *t, struct strbuf *base,
 	}
 }
 
-static int diff_tree(struct tree_desc *t1, struct tree_desc *t2,
-		     const char *base_str, struct diff_options *opt)
+static int ll_diff_tree_sha1(const unsigned char *old, const unsigned char *new,
+			     const char *base_str, struct diff_options *opt)
 {
+	struct tree_desc t1, t2;
+	void *t1tree, *t2tree;
 	struct strbuf base;
 	int baselen = strlen(base_str);
 
+	t1tree = fill_tree_descriptor(&t1, old);
+	t2tree = fill_tree_descriptor(&t2, new);
+
 	/* Enable recursion indefinitely */
 	opt->pathspec.recursive = DIFF_OPT_TST(opt, RECURSIVE);
 
@@ -159,39 +164,41 @@ static int diff_tree(struct tree_desc *t1, struct tree_desc *t2,
 		if (diff_can_quit_early(opt))
 			break;
 		if (opt->pathspec.nr) {
-			skip_uninteresting(t1, &base, opt);
-			skip_uninteresting(t2, &base, opt);
+			skip_uninteresting(&t1, &base, opt);
+			skip_uninteresting(&t2, &base, opt);
 		}
-		if (!t1->size && !t2->size)
+		if (!t1.size && !t2.size)
 			break;
 
-		cmp = tree_entry_pathcmp(t1, t2);
+		cmp = tree_entry_pathcmp(&t1, &t2);
 
 		/* t1 = t2 */
 		if (cmp == 0) {
 			if (DIFF_OPT_TST(opt, FIND_COPIES_HARDER) ||
-			    hashcmp(t1->entry.sha1, t2->entry.sha1) ||
-			    (t1->entry.mode != t2->entry.mode))
-				show_path(&base, opt, t1, t2);
+			    hashcmp(t1.entry.sha1, t2.entry.sha1) ||
+			    (t1.entry.mode != t2.entry.mode))
+				show_path(&base, opt, &t1, &t2);
 
-			update_tree_entry(t1);
-			update_tree_entry(t2);
+			update_tree_entry(&t1);
+			update_tree_entry(&t2);
 		}
 
 		/* t1 < t2 */
 		else if (cmp < 0) {
-			show_path(&base, opt, t1, /*t2=*/NULL);
-			update_tree_entry(t1);
+			show_path(&base, opt, &t1, /*t2=*/NULL);
+			update_tree_entry(&t1);
 		}
 
 		/* t1 > t2 */
 		else {
-			show_path(&base, opt, /*t1=*/NULL, t2);
-			update_tree_entry(t2);
+			show_path(&base, opt, /*t1=*/NULL, &t2);
+			update_tree_entry(&t2);
 		}
 	}
 
 	strbuf_release(&base);
+	free(t2tree);
+	free(t1tree);
 	return 0;
 }
 
@@ -206,7 +213,7 @@ static inline int diff_might_be_rename(void)
 		!DIFF_FILE_VALID(diff_queued_diff.queue[0]->one);
 }
 
-static void try_to_follow_renames(struct tree_desc *t1, struct tree_desc *t2, const char *base, struct diff_options *opt)
+static void try_to_follow_renames(const unsigned char *old, const unsigned char *new, const char *base, struct diff_options *opt)
 {
 	struct diff_options diff_opts;
 	struct diff_queue_struct *q = &diff_queued_diff;
@@ -244,7 +251,7 @@ static void try_to_follow_renames(struct tree_desc *t1, struct tree_desc *t2, co
 	diff_opts.break_opt = opt->break_opt;
 	diff_opts.rename_score = opt->rename_score;
 	diff_setup_done(&diff_opts);
-	diff_tree(t1, t2, base, &diff_opts);
+	ll_diff_tree_sha1(old, new, base, &diff_opts);
 	diffcore_std(&diff_opts);
 	free_pathspec(&diff_opts.pathspec);
 
@@ -305,23 +312,12 @@ static void try_to_follow_renames(struct tree_desc *t1, struct tree_desc *t2, co
 
 int diff_tree_sha1(const unsigned char *old, const unsigned char *new, const char *base, struct diff_options *opt)
 {
-	void *tree1, *tree2;
-	struct tree_desc t1, t2;
-	unsigned long size1, size2;
 	int retval;
 
-	tree1 = fill_tree_descriptor(&t1, old);
-	tree2 = fill_tree_descriptor(&t2, new);
-	size1 = t1.size;
-	size2 = t2.size;
-	retval = diff_tree(&t1, &t2, base, opt);
-	if (!*base && DIFF_OPT_TST(opt, FOLLOW_RENAMES) && diff_might_be_rename()) {
-		init_tree_desc(&t1, tree1, size1);
-		init_tree_desc(&t2, tree2, size2);
-		try_to_follow_renames(&t1, &t2, base, opt);
-	}
-	free(tree1);
-	free(tree2);
+	retval = ll_diff_tree_sha1(old, new, base, opt);
+	if (!*base && DIFF_OPT_TST(opt, FOLLOW_RENAMES) && diff_might_be_rename())
+		try_to_follow_renames(old, new, base, opt);
+
 	return retval;
 }