Merge branch 'ta/user-manual'

Update the user's manual to more recent versions of Git.

* ta/user-manual:
  "git prune" is safe
  Remove irrelevant reference from "Tying it all together"
  Remove unnecessary historical note from "Object storage format"
  Improve section "Merging multiple trees"
  Improve section "Manipulating branches"
  Simplify "How to make a commit"
  Fix some typos and improve wording
  Use "git merge" instead of "git pull ."
  Use current output for "git repack"
  Use current "detached HEAD" message
  Call it "Git User Manual" and remove reference to very old Git version