[PATCH/RFC] Assorted small changes to runstatus

As a regular user of git-status I'd like to propose some small changes and a
fix:

 - wording changes to the section headers

 - more context awareness in the nothing-to-commit line

 - fix runstatus breakage when amending a root commit

Additional Notes:

 - if git-reset <commit> <path> provided the ability to remove path from the
   index (if not present in commit) runstatus could advertise reset as the
   universal unstaging tool (in the non-initial-commit case)

   (this is similar to the recent request on the list to allow
    git-commit -o <path> to commit removal of path)

 - IMO git-rm without -f should not remove anything if HEAD doesn't exist
   (i.e. from the initial commit); this might be a bug