Fixed issues with SVN keyword expansion. If a file had $Id$ or $Date$ or something and the keyword was defined in that file's svn:keywords property, svn cat would return the file with the keyword expanded. This broke patching for us and essentially meant that any patches modifying anything near a keyword expansion would break.

We now collapse the keyword by turning $Keyword:.*$ into $Keyword$. We only do this for keywords actually defined in that file's svn:keywords, in order to prevent collapsing things that aren't actually defined keywords but look like them.

This also supports known keyword aliases.

Unit tests succeed! Debug output shows we're collapsing correctly.