git-gui: Refactor our ui_status_value update technique

git-gui: Refactor our ui_status_value update technique

I'm really starting to dislike global variables.  The ui_status_value
global varible is just one of those that seems to appear in a lot of
code and in many cases we didn't even declare it "global" within the
proc that updates it so we haven't always been getting all of the
updates we expected to see.

This change introduces two new global procs:

  ui_status $msg;   # Sets the status bar to show $msg.
  ui_ready;         # Changes the status bar to show "Ready."

The second (special) form is used because we often update the area
with this message once we are done processing a block of work and
want the user to know we have completed it.

I'm not fixing the cases that appear in lib/branch.tcl right now
as I'm actually in the middle of a huge refactoring of that code
to support making a detached HEAD checkout.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>