Initial rewrite of the references to Subversion on the site to Git. Rewrites

welcome! A couple of important notes:

- I've largely dropped references to Subversion altogether, even knowing that
  php-doc hasn't migrated yet. We may need to put a few more "Git/SVN account"
  references in there if php-doc isn't going to be migrated in the near future.

- I've removed the quick start hints from git-php.php after an account request
  has been sent in. The documentation there was pretty skimpy anyway and
  probably wouldn't have been enough to get a developer going, plus it's harder
  to distill the Git workflow into three or four snappy commands and I'm not
  really convinced that a form confirmation page is the right place to have
  learning materials, since it's a one-time page for most users. Instead I've
  replaced them with links to the relevant Wiki pages, which seem more likely
  to be the right place for new developers to learn.

News post to come, but I wanted to separate this out for easier review.

urlhowto.php

@@ -178,7 +178,7 @@ function a($href) {
  before the first step (PHP page search) mentioned above. Some examples
  of shortcut aliases: <tt><?php echo $MYSITE; ?>whatisphp</tt>,
  <tt><?php echo $MYSITE; ?>php4news</tt>. The latter is an external page
- alias, as it points to a file on the SVN server, containing information
+ alias, as it points to a file on the Git server, containing information
  about changes in PHP. There are also some convenient aliases, like
  <tt><?php echo $MYSITE; ?>de/phpversion</tt> which displays the German
  manual page for the phpversion() function.