gitk: Fix bug where current row number display stops working

gitk: Fix bug where current row number display stops working

The display of the current row number would stop working if the user
clicked on a line, or if selectedline got unset for any other reason,
because the trace on it got lost when it was unselected.  This fixes
it by changing the places that unset selectedline to set it to the
empty string instead, and the places that tested for it being set or
unset to compare it with the empty string.  Thus it never gets unset
now.  This actually simplified the code in a few places since it can
be compared for equality with a row number now without first testing
if it is set.

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