gitk: Add progress bars for reading in stuff and for finding

gitk: Add progress bars for reading in stuff and for finding

This uses the space formerly occupied by the find string entry field
to make a status label (unused for now) and a canvas to display a
couple of progress bars.  The bar for reading in commits is a short
green bar that oscillates back and forth as commits come in.  The
bar for showing the progress of a Find operation is yellow and advances
from left to right.

This also arranges to stop a Find operation if the user selects another
commit or pops up a context menu, and fixes the "highlight this" popup
menu items in the file list window.

Signed-off-by: Paul Mackerras <paulus@samba.org>