[merge_graph_json.py] Fix bad prefix check

Series foo should pick up foo-*.json.gz, without foobar-*.json.gz

merge_graph_json.py

@@ -31,7 +31,7 @@ def bytes(string, **kwargs):
 outdir = sys.argv[2]
 os.listdir(outdir)
 
-files = list(filter(lambda x: x != seriesname + '.json.gz' and x.startswith(seriesname) and x.endswith('.json.gz'), os.listdir(outdir)))
+files = list(filter(lambda x: x != seriesname + '.json.gz' and x.startswith(seriesname + "-") and x.endswith('.json.gz'), os.listdir(outdir)))
 files.sort()
 
 print("Merging %u files: %s" % (len(files), files))