Align section headers of 'git status' to new 'git add'.

Align section headers of 'git status' to new 'git add'.

Now that 'git add' is considered a first-class UI for 'update-index'
and that the 'git add' documentation states "Even modified files
must be added to the set of changes about to be committed" we should
make the output of 'git status' align with that documentation and
common usage.

So now we see a status output such as:

  # Added but not yet committed:
  #   (will commit)
  #
  #       new file: x
  #
  # Changed but not added:
  #   (use "git add file1 file2" to include for commit)
  #
  #       modified:   x
  #
  # Untracked files:
  #   (use "git add" on files to include for commit)
  #
  #       y

which just reads better in the context of using 'git add' to
manipulate a commit (and not a checkin, whatever the heck that is).

We also now support 'color.status.added' as an alias for the existing
'color.status.updated', as this alias more closely aligns with the
current output and documentation.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>