gitk: Fix some corner cases in the targetid/targetrow stuff

gitk: Fix some corner cases in the targetid/targetrow stuff

* Make sure targetrow is never >= numcommits
* Don't try to do anything about the target row if the targetid is
  no longer in the view; it'll just cause Tcl errors
* In insertrow, increment targetrow if we are inserting the fake
  commit at or before the target row
* In removerow, if we are removing the target row, make it the next
  one instead.

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