try-to-simplify-commit: use diff-tree --quiet machinery.

This uses diff-tree --quiet machinery to terminate the internal
diff-tree between a commit and its parents via revs.pruning (not
revs.diffopt) as soon as we find enough about the tree change.

With respect to the optionally given pathspec, we are interested
if the tree of commit is identical to the parent's, only adds
new paths to the parent's, or there are other differences.  As
soon as we find out that there is one such other kind of
difference, we do not have to compare the rest of the tree.

Because we do not call standard diff_addremove/diff_change, we
instruct the diff-tree machinery to stop early by setting
has_changes when we say we found the trees to be different.

Signed-off-by: Junio C Hamano <junkio@cox.net>

revision.c

@@ -243,6 +243,8 @@ static void file_add_remove(struct diff_options *options,
 		diff = REV_TREE_NEW;
 	}
 	tree_difference = diff;
+	if (tree_difference == REV_TREE_DIFFERENT)
+		options->has_changes = 1;
 }
 
 static void file_change(struct diff_options *options,
@@ -252,6 +254,7 @@ static void file_change(struct diff_options *options,
 		 const char *base, const char *path)
 {
 	tree_difference = REV_TREE_DIFFERENT;
+	options->has_changes = 1;
 }
 
 int rev_compare_tree(struct rev_info *revs, struct tree *t1, struct tree *t2)
@@ -261,6 +264,7 @@ int rev_compare_tree(struct rev_info *revs, struct tree *t1, struct tree *t2)
 	if (!t2)
 		return REV_TREE_DIFFERENT;
 	tree_difference = REV_TREE_SAME;
+	revs->pruning.has_changes = 0;
 	if (diff_tree_sha1(t1->object.sha1, t2->object.sha1, "",
 			   &revs->pruning) < 0)
 		return REV_TREE_DIFFERENT;
@@ -285,6 +289,7 @@ int rev_same_tree_as_empty(struct rev_info *revs, struct tree *t1)
 	empty.size = 0;
 
 	tree_difference = REV_TREE_SAME;
+	revs->pruning.has_changes = 0;
 	retval = diff_tree(&empty, &real, "", &revs->pruning);
 	free(tree);
 
@@ -552,6 +557,7 @@ void init_revisions(struct rev_info *revs, const char *prefix)
 	revs->ignore_merges = 1;
 	revs->simplify_history = 1;
 	revs->pruning.recursive = 1;
+	revs->pruning.quiet = 1;
 	revs->pruning.add_remove = file_add_remove;
 	revs->pruning.change = file_change;
 	revs->lifo = 1;