These are in fact two different bugs on the same code:
a) Deletion summary was prefilled with up to 150 characters from the page content, leaving space for the sysop to add his reason. r27170 changed it to the maximun available (allowed by the language), eating that content. Rationale was "In the deletion default reasons, calculate how much text to get from the article text, rather than getting 150 characters (which may be too much)". I don't know why max(150, $maxLength) wasn't used. (Regression was complained onm ML).