gitk: Fix bugs in the Find function

gitk: Fix bugs in the Find function

This fixes the problem reported by Brian Downing where searching for
a string that doesn't exist would give a Tcl error.  The basic problem
was that we weren't reading the data for the last commit since it
wasn't terminated with a null.  This effectively adds a null on the end
(if there isn't one already) to make sure we process the last commit.

This also makes the yellow background behind instances of the search
string appear more consistently, and fixes a bug where the "/" key
would just find the same commit again and again instead of advancing.

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