gitk: Limit how often we change the canvas scrolling region

gitk: Limit how often we change the canvas scrolling region

For some unknown reason, changing the scrolling region on the canvases
provokes multiple milliseconds worth of computation in the X server,
and this can end up slowing gitk down significantly.  This works around
the problem by limiting the rate at which we update the scrolling region
after the first 100 rows to at most 2 per second.

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