gitk: Make line origin search update the busy status

gitk: Make line origin search update the busy status

Currently the 'show origin of this line' feature does not update the
status field of the gitk window, so it is not evident that any
processing is going on.  It may seem at first that clicking the item
had no effect.

This commit adds calls to set and clear the busy status with an
appropriate title, similar to other search commands.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>