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>

version/mkversion/mkversion.go

@@ -419,7 +419,7 @@ func infoFromDir(dir string) (verInfo, error) {
 	if err != nil {
 		return verInfo{}, err
 	}
-	date, err := r.output("git", "log", "-n1", "--format=%%ct", "HEAD")
+	date, err := r.output("git", "log", "-n1", "--format=%ct", "HEAD")
 	if err != nil {
 		return verInfo{}, err
 	}