fast-export: Correctly generate initial commits with no parents

If we are exporting a commit which has no parents we may be doing it to a branch that already exists, causing fast-import to assume the branch's current revision should be the sole parent of the new commit. This can cause git fast-export | git fast-import to produce an incorrect graph for:

A-------M----o------o refs/heads/master

          /
       B-+

In this graph A and B are initial commits (no parents) but if A was output first to refs/heads/master and then B is output fast-import would assume the graph was this instead:

A-------M----o------o refs/heads/master

    \     /
     +-B-+

Which would cause B, M, and all later commits to have a different SHA-1, and obviously be quite a different graph.

Sending a reset command prior to B informs fast-import to clear the implied parent of A, allowing B to remain an initial commit.

---
 builtin-fast-export.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

index 1dfc01e..d0a462f 100644
@@ -188,6 +188,8 @@ static void handle_commit(struct commit *commit, struct rev_info *rev)
 	mark_object(&commit->object);
 	if (!is_encoding_utf8(encoding))
 		reencoded = reencode_string(message, "UTF-8", encoding);
+	if (!commit->parents)
+		printf("reset %s\n", (const char*)commit->util);
 	printf("commit %s\nmark :%d\n%.*s\n%.*s\ndata %u\n%s",
 	       (const char *)commit->util, last_idnum,
 	       (int)(author_end - author), author,

1.5.6.rc2.165.g2a0cc