Merge gitk changes from Paul Mackerras at git://ozlabs.org/~paulus/gitk

* git://ozlabs.org/~paulus/gitk:
  gitk: Teach gitk to respect log.showroot
  gitk: Add menu items for comparing a commit with the marked commit
  gitk: Speed up resolution of short SHA1 ids
  gitk: Use symbolic font names "sans" and "monospace" when available
  gitk: Skip over AUTHOR/COMMIT_DATE when searching all fields
  gitk: Make "git describe" output clickable, too
  gitk: Fix the display of files when filtered by path
  gitk: Use a tabbed dialog to edit preferences
  gitk: Use "gitk: repo-top-level-dir" as window title