Fix: Editing the commit message changes the author name and date

When the commit message is edited in the UI, the author name and
timestamp changes to the currently logged in user and current time.

The author name and date would not be changed by simply uploading a
new patch set (unless the committer amended with the --author option)
so neither should editing the commit message.  Set the committer to
the currently logged in user, and leave the author as the original
author. This matches what git commit --amend would do.

When creating the new patch set, match behavior with ReceiveCommits by
setting the created on field to now, and the uploader to the currently
logged in user.

Now downloading a commit, amending it locally and pushing it back
has the same results as editing it online through the web UI.

Found-by: David Pursehouse <david.pursehouse@sonymobile.com>
Change-Id: Icb697f623106563cd8c12cb52ef36d2c09bb4fc9