Merge branch 'tm/fetch-prune'

Fetching 'frotz' branch with "git fetch", while having
'frotz/nitfol' remote-tracking branch from an earlier fetch, would
error out, primarily because the command has not been told to
remove anything on our side. In such a case, "git fetch --prune"
can be used to remove 'frotz/nitfol' to make room to fetch and
store 'frotz' remote-tracking branch.

* tm/fetch-prune:
  fetch --prune: Run prune before fetching
  fetch --prune: always print header url