(Bug 49034) Normalize modified links during serialization.

* When a regular piped link is edited, the serializer should
  treat it as modified and normalize its output to a simple
  link when possible.

* No change in parser test results.

* Tested as follows:
--------------------------------------------
[subbu@earth tests] cat /tmp/old.html
<p data-parsoid='{"dsr":[0,11,0,0]}'><a rel="mw:WikiLink" href="./Foo"
data-parsoid='{"tsr":[0,11],"a":{"href":"./Foo"},"sa":{"href":"Foo"},"stx":"piped","dsr":[0,11,6,2]}'>bar</a></p>
[subbu@earth tests] cat /tmp/new.html
<p data-parsoid='{"dsr":[0,11,0,0]}'><a rel="mw:WikiLink" href="./Foo"
data-parsoid='{"tsr":[0,11],"a":{"href":"./Foo"},"sa":{"href":"Foo"},"stx":"piped","dsr":[0,11,6,2]}'>Foo</a></p>
[subbu@earth tests] node parse --html2wt --selser --oldtext "[[Foo|bar]]" --oldhtmlfile /tmp/old.html < /tmp/new.html
[[Foo]]
[subbu@earth tests] node parse --html2wt < /tmp/new.html
[[Foo|Foo]]
--------------------------------------------

* TODO: Need a way to improve our selser change generation script
  to do more deterministic changes to capture test cases like this.

Change-Id: I6abef05522192f667d483b4eb4e05de771496c28
Merge "(Bug 49034) Normalize modified links during serialization."