log --graph --left-right: show left/right information in place of '*'

log --graph --left-right: show left/right information in place of '*'

With the --graph option, the graph already outputs 'o' instead of '*'
for boundary commits.  Make it emit '<' or '>' when --left-right is
specified.

(This change also disables the '^' prefix for UNINTERESTING commits.
The graph code currently doesn't print anything special for these
commits, since it assumes no UNINTERESTING, non-BOUNDARY commits are
displayed.  This is potentially a bug if UNINTERESTING non-BOUNDARY
commits can actually be displayed via some code path.)

[jc: squashed the left-right change from Dscho and Adam's fixup into one]

Signed-off-by: Adam Simpkins <adam@adamsimpkins.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>