builtin-fast-export: Only output a single parent per line

According to the git-fast-import man-page, you can only put a single
committish per merge: line, like this:

  merge :10
  merge :11

However, git-fast-export puts all parents on a single line, like this:

  merge :10 :11

This changes fast-export to output a single parent per line.  Otherwise
neither git-fast-import nor bzr-fast-import can read its output.

[jc: fix-up to remove excess LF in the output that makes fast-import barf]

Signed-off-by: Pieter de Bie <pdebie@ai.rug.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fast-export.c

@@ -205,14 +205,10 @@ static void handle_commit(struct commit *commit, struct rev_info *rev)
 			continue;
 		if (i == 0)
 			printf("from :%d\n", mark);
-		else if (i == 1)
-			printf("merge :%d", mark);
 		else
-			printf(" :%d", mark);
+			printf("merge :%d\n", mark);
 		i++;
 	}
-	if (i > 1)
-		printf("\n");
 
 	log_tree_diff_flush(rev);
 	rev->diffopt.output_format = saved_output_format;