[Fix #1441] Compare AST for whole buffer in AutocorrectUnlessChangingAST

We were already re-parsing the whole buffer to check for syntax
errors. If we check for AST equality instead, we don't need to
check the syntax.
Merge pull request #1443 from jonas054/1441_blocks

[Fix #1441] Compare AST for whole buffer in AutocorrectUnlessChangingAST