gitk: Use git log and add support for --left-right

gitk: Use git log and add support for --left-right

This is based on patches from Linus Torvalds and Junio Hamano, so the
ideas here are theirs.

This makes gitk use "git log -z --pretty=raw" instead of "git rev-list"
to generate the list of commits, and also makes it grok the "<" and ">"
markers that git log (and git rev-list) output with the --left-right
flag to indicate which side of a symmetric diff a commit is reachable
from.  Left-side commits are drawn with a triangle pointing leftwards
instead of a circle, and right-side commits are drawn with a triangle
pointing rightwards.  The commitlisted list is used to store the
left/right information as well as the information about whether each
commit is on the boundary.

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