gitk: Highlight current search hit in orange

gitk: Highlight current search hit in orange

When searching for text in the diff, and there are multiple occurrences
of the search string, the current one is highlighted in orange, and the
other ones in yellow. This makes it much easier to understand what happens
when you then click the Search button or hit Ctrl-S repeatedly.

Signed-off-by: Stefan Haller <stefan@haller-berlin.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>