dashboard/watcher: use commit date, not author date

Commit date is what is relevant to the dashboard.

Change-Id: I807492768c8f7dcbf9ee6bdc233976cb25ee8647
Reviewed-on: https://go-review.googlesource.com/1356
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

watcher/watcher.go

@@ -505,7 +505,7 @@ func (r *Repo) remotes() ([]string, error) {
 const logFormat = `--format=format:%H
 %P
 %an <%ae>
-%aD
+%cD
 %B
 ` + logBoundary