gitk: Fix problem with target row not being in scroll region

gitk: Fix problem with target row not being in scroll region

Since we limit the rate at which we do updates to the canvas scrolling
regions, it's possible to get into selectline for a row that is
outside the currently-set scrolling region.  When this happens,
selectline can't scroll to show the selected line, and as a
consequence, drawvisible chooses some other bogus row to be the
target row.

This fixes it by calling setcanvscroll from selectline in this case.
We also set selectedline (and currentid) before calling drawvisible
so that drawvisible makes the right choice of target row.

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