Optimize S_find_first_nonspace.

We were needlessly redoing things we'd already done.
Now we skip the work if the first nonspace is greater
than the current offset.

This fixes pathological slowdown with deeply nested
lists (commonmark#255).  For N = 3000, the time goes from over
17s to about 0.7s.

Thanks to @mity for diagnosing the problem.

Author: jgm

src/blocks.c

@@ -615,22 +615,24 @@ static void S_find_first_nonspace(cmark_parser *parser, cmark_chunk *input) {
   char c;
   int chars_to_tab = TAB_STOP - (parser->column % TAB_STOP);
 
-  parser->first_nonspace = parser->offset;
-  parser->first_nonspace_column = parser->column;
-  while ((c = peek_at(input, parser->first_nonspace))) {
-    if (c == ' ') {
-      parser->first_nonspace += 1;
-      parser->first_nonspace_column += 1;
-      chars_to_tab = chars_to_tab - 1;
-      if (chars_to_tab == 0) {
+  if (parser->first_nonspace <= parser->offset) {
+    parser->first_nonspace = parser->offset;
+    parser->first_nonspace_column = parser->column;
+    while ((c = peek_at(input, parser->first_nonspace))) {
+      if (c == ' ') {
+        parser->first_nonspace += 1;
+        parser->first_nonspace_column += 1;
+        chars_to_tab = chars_to_tab - 1;
+        if (chars_to_tab == 0) {
+          chars_to_tab = TAB_STOP;
+        }
+      } else if (c == '\t') {
+        parser->first_nonspace += 1;
+        parser->first_nonspace_column += chars_to_tab;
         chars_to_tab = TAB_STOP;
+      } else {
+        break;
       }
-    } else if (c == '\t') {
-      parser->first_nonspace += 1;
-      parser->first_nonspace_column += chars_to_tab;
-      chars_to_tab = TAB_STOP;
-    } else {
-      break;
     }
   }
 
@@ -1160,6 +1162,9 @@ static void S_process_line(cmark_parser *parser, const unsigned char *buffer,
 
   parser->offset = 0;
   parser->column = 0;
+  parser->first_nonspace = 0;
+  parser->first_nonspace_column = 0;
+  parser->indent = 0;
   parser->blank = false;
   parser->partially_consumed_tab = false;