Changeset: Return a new op object by default when iterating

Reusing the same op object for each iteration can result in very weird
behaviors because previously yielded op objects will get a surprise
mutation.

It is unclear why the code was written to reuse the same object. There
was no comment, nor is there a commit message providing rationale (it
has behaved this way since the very first commit). Perhaps the objects
were reused to improve performance (fewer object allocations that need
to be garbage collected). I do expect this change to reduce
performance somewhat, but not enough to warrant reverting this commit.

src/static/js/Changeset.js

@@ -126,10 +126,9 @@ exports.opIterator = (opsStr, optStartIndex) => {
     return result;
   };
   let regexResult = nextRegexMatch();
-  const obj = exports.newOp();
 
-  const next = (optObj) => {
-    const op = (optObj || obj);
+  const next = (optOp) => {
+    const op = optOp || exports.newOp();
     if (regexResult[0]) {
       op.attribs = regexResult[1];
       op.lines = exports.parseNum(regexResult[2] || 0);