post-receive-hook: Remove the From field from the generated email header so that...

post-receive-hook: Remove the From field from the generated email header so that the pusher's name is used

Using the name of the committer of the revision at the tip of the
updated ref is not sensible.  That information is available in the email
itself should it be wanted, and by supplying a "From", we were
effectively hiding the person who performed the push - which is useful
information in itself.

Signed-off-by: Andy Parkins <redacted>
Signed-off-by: Junio C Hamano <redacted>