Add warning if user-configured hgignore file isn't found

Implementation of per-user .hgignore. Reference: http://www.selenic.com/mercurial/bts/issue166 If the [ui] section of .hgrc contains keys like "ignore" or "ignore.something", the values corresponding to these keys are treated as per-user hgignore files. These hgignore files apply to all repositories used by that user.