gitk: Fix a couple of bugs in the find function

gitk: Fix a couple of bugs in the find function

First, findmore would sometimes get a Tcl error due to relying on
varcorder and vrownum having valid values for the rows being searched,
but they may not be valid unless update_arcrows is called, so this
makes findmore call update_arcrows if necessary.

Secondly, in the "touching paths" and "adding/removing string" modes,
findmore was treating fhighlights($row) == -1 as meaning the row
matches, whereas it only means that we haven't received an answer from
the external git diff-tree process about it yet.  This fixes it.

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