Fix mark_parents() to account for bad luck traversals

If commit timestamps are off, we're more likely to hit a traversal
where the first path ends up traversing past the root commit of the tree.
If that happens, it's possible that the loop will complete before the second
path marks some of those final parents. This fix keeps track of the root
nodes that are encountered in the traversal, and verify that they are
properly marked.

In the best case, with accurate timestamps, the traversal will continue
to terminate when all the commits are STALE (parents of a merge-base), as
it did before. In the worst case, where one path makes a complete traversal
past a root commit, we will continue the loop until the root commit itself
is marked.

src/graph.c

@@ -10,7 +10,7 @@
 #include "merge.h"
 #include "git2/graph.h"
 
-static int interesting(git_pqueue *list)
+static int interesting(git_pqueue *list, git_commit_list *roots)
 {
 	unsigned int i;
 	/* element 0 isn't used - we need to start at 1 */
@@ -20,6 +20,12 @@ static int interesting(git_pqueue *list)
 			return 1;
 	}
 
+	while(roots) {
+		if ((roots->item->flags & STALE) == 0)
+			return 1;
+		roots = roots->next;
+	}
+
 	return 0;
 }
 
@@ -28,6 +34,7 @@ static int mark_parents(git_revwalk *walk, git_commit_list_node *one,
 {
 	int error;
 	unsigned int i;
+	git_commit_list *roots = NULL;
 	git_pqueue list;
 
 	/* if the commit is repeated, we have a our merge base already */
@@ -52,11 +59,13 @@ static int mark_parents(git_revwalk *walk, git_commit_list_node *one,
 		return -1;
 
 	/* as long as there are non-STALE commits */
-	while (interesting(&list)) {
+	while (interesting(&list, roots)) {
 		git_commit_list_node *commit;
 		int flags;
 
 		commit = git_pqueue_pop(&list);
+		if (commit == NULL)
+			break;
 
 		flags = commit->flags & (PARENT1 | PARENT2 | STALE);
 		if (flags == (PARENT1 | PARENT2)) {
@@ -78,8 +87,15 @@ static int mark_parents(git_revwalk *walk, git_commit_list_node *one,
 			if (git_pqueue_insert(&list, p) < 0)
 				return -1;
 		}
+
+		if (commit->out_degree == 0) {
+			if (git_commit_list_insert(commit, &roots) == NULL)
+				return -1;
+		}
 	}
 
+	if (roots)
+		git_commit_list_free(&roots);
 	git_pqueue_free(&list);
 
 	return 0;