The message gives a detailed explanation of the commit the requester based
the changes on, but lacks information that is necessary for the person who
performs a fetch & merge in order to verify that the correct branch was
fetched when responding to the pull request.

Add a few more lines to describe the commit at the tip expected to be
fetched to the same level of detail as the base commit.

Also update the warning message slightly when the script notices that the
commit may not have been pushed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>