graph API: fix bug in graph_is_interesting()

Previously, graph_is_interesting() did not behave quite the same way as
the code in get_revision().  As a result, it would sometimes think
commits were uninteresting, even though get_revision() would return
them.  This resulted in incorrect lines in the graph output.

This change creates a get_commit_action() function, which
graph_is_interesting() and simplify_commit() both now use to determine
if a commit will be shown.  It is identical to the old simplify_commit()
behavior, except that it never calls rewrite_parents().

This problem was reported by Santi Béjar.  The following command
would exhibit the problem before, but now works correctly:

  git log --graph --simplify-by-decoration --oneline v1.6.3.3

Previously git graph did not display the output for this command
correctly between f29ac4f and 66996ec, among other places.

Signed-off-by: Adam Simpkins <simpkins@facebook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -165,6 +165,7 @@ enum commit_action {
 	commit_error
 };
 
+extern enum commit_action get_commit_action(struct rev_info *revs, struct commit *commit);
 extern enum commit_action simplify_commit(struct rev_info *revs, struct commit *commit);
 
 #endif