fast-export: ensure we traverse commits in topological order

fast-export will only list as parents those commits which have already
been traversed (making it appear as if merges have been squashed if not
all parents have been traversed).  To avoid this silent squashing of
merge commits, we request commits in topological order.

Signed-off-by: Elijah Newren <[email protected]>
Acked-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

builtin-fast-export.c

@@ -511,6 +511,7 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 
 	get_tags_and_duplicates(&revs.pending, &extra_refs);
 
+	revs.topo_order = 1;
 	if (prepare_revision_walk(&revs))
 		die("revision walk setup failed");
 	revs.diffopt.format_callback = show_filemodify;