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>

pretty.c

@@ -859,11 +859,7 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 		                              c->abbrev_parent_hashes.off;
 		return 1;
 	case 'm':		/* left/right/bottom */
-		strbuf_addch(sb, (commit->object.flags & BOUNDARY)
-		                 ? '-'
-		                 : (commit->object.flags & SYMMETRIC_LEFT)
-		                 ? '<'
-		                 : '>');
+		strbuf_addstr(sb, get_revision_mark(NULL, commit));
 		return 1;
 	case 'd':
 		format_decoration(sb, commit);