Merge branch 'ab/fetch-tags-noclobber'

The rules used by "git push" and "git fetch" to determine if a ref
can or cannot be updated were inconsistent; specifically, fetching
to update existing tags were allowed even though tags are supposed
to be unmoving anchoring points.  "git fetch" was taught to forbid
updates to existing tags without the "--force" option.
This is a backward incompatible change but in a good way; it may
still need to be treated carefully.

* ab/fetch-tags-noclobber:
  fetch doc: correct grammar in --force docs
  push doc: add spacing between two words