Reimplement emailing part of hooks--update in contrib/hooks/post-receive-email

The update hook is no longer the correct place to generate emails; there
is now the hooks/post-receive script which is run automatically after a
ref has been updated.

This patch is to make use of that new location, and to address some
faults in the old update hook.

The primary problem in the conversion was that in the update hook, the
ref has not actually been changed, but is about to be.  In the
post-receive hook the ref has already been updated.  That meant that
where we previously had lines like:

 git rev-list --not --all

would now give the wrong list because "--all" in the post-receive hook
includes the ref that we are making the email for.  This made it more
difficult to show only the new revisions added by this update.

The solution is not pretty; however it does work and doesn't need any
changes to git-rev-list itself.  It also fixes (more accurately: reduces
the likelihood of) a nasty race when another update occurs while this
script is running.  The solution, in short, looks like this (see the
source code for a longer explanation)

 git rev-parse --not --all | grep -v $(git rev-parse $refname) |
 git rev-list --pretty --stdin $oldrev..$newrev

This uses git-rev-parse followed by grep to filter out the revision of
the ref in question before it gets to rev-list and inhibits the output
of itself.  By using $(git rev-parse $revname) rather than $newrev as the
filter, it also takes care of the situation where another update to the
same ref has been made since $refname was $newrev.

The second problem that is addressed is that of tags inhibiting the
correct output of an update email.  Consider this, with somebranch and
sometag pointing at the same revision:

 git push origin somebranch
 git push origin sometag

That would work fine; the push of the branch would generate an email
containing all the new commits introduced by the update, then the push
of the tag would generate the shortlog formatted tag email.  Now
consider:

 git push origin sometag
 git push origin somebranch

When some branch comes to run its "--not --all" line, it will find
sometag, and filter those commits from the email - leaving nothing.
That meant that those commits would not show (in full) on any email.
The fix is to not use "--all", and instead use "--branches" in the
git-rev-parse command.

Other changes
 * Lose the monstrous one-giant-script layout and put things in easy to
   digest functions.  This makes it much easier to find the place you
   need to change if you wanted to customise the output.  I've also
   tried to write more verbose comments for the same reason.  The hook
   script is big, mainly because of all the different cases that it has
   to handle, so being easy to navigate is important.
 * All uses of "git-command" changed to "git command", to cope better
   if a user decided not to install all the hard links to git;
 * Cleaned up some of the English in the email
 * The fact that the receive hook makes the ref available also allows me
   to use Shawn Pearce's fantastic suggestion that an annotated tag can
   be parsed with git-for-each-ref.  This removes the potentially
   non-portable use of "<<<" heredocs and the nasty messing around with
   "date" to convert numbers of seconds UTC to a real date
 * Deletions are now caught and notified (briefly)
 * To help with debugging, I've retained the command line mode from the
   update hook; but made it so that the output is not emailed, it's just
   printed to the screen.  This could then be redirected if the user
   wanted
 * Removed the "Hello" from the beginning of the email - it's just
   noise, and no one seriously has their day made happier by "friendly"
   programs
 * The fact that it doesn't rely on repository state as an indicator any
   more means that it's far more stable in its output; hopefully the
   same arguments will always generate the same email - even if the
   repository changes in the future.  This means you can easily recreate
   an email should you want to.
 * Included Jim Meyering's envelope sender option for the sendmail call
 * The hook is now so big that it was inappropriate to copy it
   to every repository by keeping it in the templates directory.
   Instead, I've put a comment saying to look in contrib/hooks, and
   given an example of calling the script from that template hook.  The
   advantage of calling the script residing at some fixed location is
   that if a future package of git included a bug fixed version of the
   script, that would be picked up automatically, and the user would not
   have to notice and manually copy the new hook to every repository
   that uses it.

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

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