request-pull: use the real fork point when preparing the message

The command takes the "start" argument and computes the merge base
between it and the commit to be pulled so that we can show the diffstat,
but uses the "start" argument as-is when composing the message

    The following changes since commit $X are available

to tell the integrator which commit the work is based on. Giving "origin"
(most of the time it resolves to refs/remotes/origin/master) as the start
argument is often convenient, but it is usually not the fork point, and
does not help the integrator at all.

Use the real fork point, which is the merge base we already compute, when
composing that part of the message.

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

git-request-pull.sh

@@ -96,7 +96,7 @@ git show -s --format='The following changes since commit %H:
   %s (%ci)
 
 are available in the git repository at:
-' $baserev &&
+' $merge_base &&
 echo "  $url${ref+ $ref}" &&
 git show -s --format='
 for you to fetch changes up to %H: