mercurial.ini: provide more useful info
* Add warning about not editing the site-wide file on Vista
* More detail about where to create user Mercurial.ini
* Provide examples for ssh tools
* List extensions, all disabled
* add [patch] section, for fixing patch behavior when using win32text
* example [extdiff] section, all disabled
* example [diff] section, all disabled
Backed out changeset
5724cd7b3688 (depends on other changes, no tests)