R1:451610949515

Survive hand-crafted Git commits which are missing timestamp information

Summary:
Fixes T12062. Like the commits from the year 3500, you can artificially build commits with no date information.

We could explicitly store these as `null` to fully respect the underlying datastore. However, I think it's very unlikely that these commits are intentional/meaningful or that this is valuable.

Additionally, "git show" interprets these commits as "Jan 1, 1970". Just store a `0` to mimic its behavior.

Test Plan…