gitk: Get rid of lookingforhead, use commitinterest instead

gitk: Get rid of lookingforhead, use commitinterest instead

Now that we have a general-purpose way of taking some action when a
commit ID of interest is encountered, use that for triggering the
git diff-index process when we find the currently checked-out head,
rather than the special-purpose lookingforhead variable.

Also do the commitinterest processing in getcommitlines rather than
in showstuff.

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