gitk: Fix another collection of bugs

gitk: Fix another collection of bugs

* Fixed a bug that occasionally resulted in Tcl "can't use empty string
  as argument to incr" errors - rowofcommit was sometimes not calling
  update_arcrows when it needed to.

* Fixed a "no such element in array" error when removing a fake row,
  by unsetting currentid and selectedline in removerow if the row we
  are removing is the currently selected row.

* Made the "update commits" function always do "reread references".

* Made dodiffindex et al. remove the fake row(s) if necessary.

* Fixed a bug where clicking on a row in the graph display pane didn't
  account for horizontal scrolling of the pane.

* Started changing things that cached information based on row numbers
  to use commit IDs instead -- this converts the "select line" items
  that are put into the history list to use "select by ID" instead.

* Simplified redrawtags a bit, and fixed a bug where it would use the
  mainfont for working out how far it extends to the right in the graph
  display pane rather than the actual font (which might be bold).

* Fixed a bug where "reread references" wouldn't notice if the currently
  checked-out head had changed.

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