Only show log entries for new revisions in hooks--update

If you were issuing emails for two branches, and one merged the other,
you would get the same log messages appearing in two separate emails.

e.g. A working repository, where the last push to central was done at
     the revision marked "B", after which two branches were developed
     further.

  * -- B -- 1 -- 1 -- M (branch1)
        \           /
         2 -- 2 -- 2 (branch2)

Now imagine that branch2 is pushed to the email-generating repository;
an email containing all the "2" revisions would be sent.  Now, let's say
branch1 is pushed, the old update hook would run

 git-rev-list $newrev ^$baserev

Where $newrev would be "M" and $baserev would be "B".  This list
includes all the "2" revisions as well as all the "1" revisions.

This patch addresses this problem by using

 git-rev-parse --not --all | git-rev-list --stdin $newrev ^$baserev

To inhibit the display of all revisions that are already in the
repository.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

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