Fix timestamp output format in perfdiag.
Fix timestamp output format in perfdiag.

chriskuehl/gsutil-debian,craigcitro/gsutil,mattdr/gsutil,GoogleCloudPlatform/gsutil,GoogleCloudPlatform/gsutil,BrandonY/gsutil,ttiurani/gsutil,dtjackson/gsutil,dimfeld/gsutil,fishjord/gsutil