add timestamps to unused log files

This will be used in expiring old unused objects. The timestamp is when it
was first noticed it was unused.

Backwards compatability: It supports reading old format unused log files.
The old version of git-annex will ignore lines in log files written by the
new version, so the worst interop problem would be git annex dropunused not
knowing some numbers that git-annex unused reported.

0 files changed, 0 insertions, 0 deletions