Patch by MinuteElectron - https://bugzilla.wikimedia.org/attachment.cgi?id=4772 with fixes
Replaces an instance variable with a constant, since it doesn't change, which is safe to access when the function is called statically from an outside class, as some extensions do.
* Verified Preprocessor_Hash against Preprocessor_DOM with a differential fuzz test.
* Improved the utility of Parser_DiffTest by synchronising strip markers and allowing for loose object comparison.
* Abstracted PPDStack to make it usable in Preprocessor_Hash.
The implications in general are that headings inside properly-closed double braces will not have section edit links, even where the braces are broken by invalid title characters and expanded literally to recover the heading. Respecting such headings as section breaks would run into the same problems as those discussed w.r.t. LST on wikitech-l -- there's no way to recover the text for a section that starts on one tree level and finishes on a different one. The fact that higher level <h> nodes appear in the XML is just due to a shortcut in preprocessToObj(): headings are put into the stack in case the braces turn out to be unclosed, in which case the heading will be at level 1 of the tree.
* Made ==foo==<!----> create a valid section edit link
* Changed header processing heuristics -- now double-equals signs are generally respected as header starts, and will break template invocations, and single equals signs are respected as header syntax but might not generate a section edit link.
* Stabilise timestamps generated by the parser to avoid diff test false positives
* Fixed msgnw bug. Use RECOVER_ORIG.
* Fixed editintro bug. Cloning the parser in MessageCache has some side-effects that need to be corrected.
* Fixed typo in Parser_DiffTest.php
* General improvements to preprocessorFuzzTest.php
* Fixed breakage of XML output feature in Special:ExpandTemplates
* Reduced stack depth by using an internal stack in expand(), and by having some common code paths (e.g. non-subst double-brace during PST) return objects which can be expanded in that internal stack instead of the PHP stack. This is friendly to xdebug but slightly slower than the original version. Also it probably helps robustness when you don't add 7 stack levels per pair of double braces.
* Profiling indicates that expand and PPD are now good targets for porting to C. Abstracted and refactored the relevant code to allow for a drop-in replacement. A factor of 2 reduction in average-case replaceVariables() time may be possible.
* Verified with preprocessorFuzzTest.php against r29950, updated to allow better PST tests.
* Made parserTests.php respect $wgParserConf
* LST and ParserFunctions need a simultaneous update with the core due to changed interfaces. DOM objects are now wrapped rather than directly exposed.
