version/mkversion: fix commit date lookup in the tailscale.com checkout path

infoFromDir passed --format=%%ct to git log, which git renders as the
literal string "%ct", so VersionInfo.GitDate was never a timestamp for
builds made directly from a tailscale.com checkout. The corp path in
infoFromCache already used the correct %ct.

Updates tailscale/corp#44945

Change-Id: I28d193019a8dd3d2bdd165da5e4584df1964afd6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f