revision: allow setting custom limiter function

This commit enables users of `struct rev_info` to peform custom limiting
during a revision walk (i.e. `get_revision`).

If the field `include_check` has been set to a callback, this callback
will be issued once for each commit before it is added to the "pending"
list of the revwalk. If the include check returns 0, the commit will be
marked as added but won't be pushed to the pending list, effectively
limiting the walk.

Signed-off-by: Vicent Marti <tanoku@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -779,6 +779,10 @@ static int add_parents_to_list(struct rev_info *revs, struct commit *commit,
 		return 0;
 	commit->object.flags |= ADDED;
 
+	if (revs->include_check &&
+	    !revs->include_check(commit, revs->include_check_data))
+		return 0;
+
 	/*
 	 * If the commit is uninteresting, don't try to
 	 * prune parents - we want the maximal uninteresting

revision.h

@@ -168,6 +168,8 @@ struct rev_info {
 	unsigned long min_age;
 	int min_parents;
 	int max_parents;
+	int (*include_check)(struct commit *, void *);
+	void *include_check_data;
 
 	/* diff info for patches and for paths limiting */
 	struct diff_options diffopt;