Use PathFilterGroup in LogServlet

PathFilterGroup has some short-circuiting behavior that PathFilter
does not. This cuts time for log of Chromium's
chrome/browser/chrome_browser_main.cc from 12s to 8s on my
workstation.

This is low-hanging fruit. JGit's log still performs very badly; the
same file logs in under a second with C git.

Change-Id: Ief0f81e87273b95fabe476418cda6b3c5851f449