Merge branch 'mv/log-since-as-filter'

"git log --since=X" will stop traversal upon seeing a commit that
is older than X, but there may be commits behind it that is younger
than X when the commit was created with a faulty clock.  A new
option is added to keep digging without stopping, and instead
filter out commits with timestamp older than X.

* mv/log-since-as-filter:
  log: "--since-as-filter" option is a non-terminating "--since" variant