rev-list/log: factor out revision mark generation

rev-list/log: factor out revision mark generation

Currently, we have identical code for generating revision marks ('<',
'>', '-') in 5 places.

Factor out the code to a single function get_revision_mark() for easier
maintenance and extensibility.

Note that the check for !!revs in graph.c (which gets removed
effectively by this patch) is superfluous.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>