Merge branch 'maint'

* maint:
  git-apply: fix whitespace stripping
  apply --index-info: fall back to current index for mode changes
  core-tutorial: minor cleanup
  documentation: replace Discussion section by link to user-manual chapter
  user-manual: todo updates and cleanup
  user-manual: fix introduction to packfiles
  user-manual: move packfile and dangling object discussion
  user-manual: rewrite object database discussion
  user-manual: reorder commit, blob, tree discussion
  user-manual: rewrite index discussion
  user-manual: create new "low-level git operations" chapter
  user-manual: rename "git internals" to "git concepts"
  user-manual: move object format details to hacking-git chapter
  user-manual: adjust section levels in "git internals"
  revision walker: --cherry-pick is a limited operation
  git-sh-setup: typofix in comments