gitk: Don't filter view arguments through git rev-parse

gitk: Don't filter view arguments through git rev-parse

Previously we passed the arguments indicating what commits the user
wants to view through git rev-parse to get a list of IDs (positive and
negative), then gave that to git log.  This had a couple of problems,
notably that --merge and --left-right didn't get handled properly.

Instead we now just pass the original arguments to git log.  When doing
an update, we append --not followed by the list of commits we have seen
that have no children, since we have got (or will get) their ancestors
from the first git log.  If the first git log isn't finished yet, we
might get some duplicates from the second git log, but that doesn't
cause any problem.

Also get rid of the unused vnextroot variable.

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