gitk: Show current row number and total number of rows

gitk: Show current row number and total number of rows

This adds a couple of fields in the bar just below the upper panes
that show the row number of the currently selected commit, and how
many rows are displayed in total.  The latter increments as commits
are read in, and thus functions to show that progress is being made.
This therefore also removes the code that showed progress using a
green oscillating bar in the progress bar window (which some people
disliked).

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