transport: drop support for git-over-rsync

The git-over-rsync protocol is inefficient and broken, and
has been for a long time. It transfers way more objects than
it needs (grabbing all of the remote's "objects/",
regardless of which objects we need). It does its own ad-hoc
parsing of loose and packed refs from the remote, but
doesn't properly override packed refs with loose ones,
leading to garbage results (e.g., expecting the other side
to have an object pointed to by a stale packed-refs entry,
or complaining that the other side has two copies of the
refs[1]).

This latter breakage means that nobody could have
successfully pulled from a moderately active repository
since cd547b4 (fetch/push: readd rsync support, 2007-10-01).

We never made an official deprecation notice in the release
notes for git's rsync protocol, but the tutorial has marked
it as such since 914328a (Update tutorial., 2005-08-30).
And on the mailing list as far back as Oct 2005, we can find
Junio mentioning it as having "been deprecated for quite
some time."[2,3,4]. So it was old news then; cogito had
deprecated the transport in July of 2005[5] (though it did
come back briefly when Linus broke git-http-pull!).

Of course some people professed their love of rsync through
2006, but Linus clarified in his usual gentle manner[6]:

  > Thanks!  This is why I still use rsync, even though
  > everybody and their mother tells me "Linus says rsync is
  > deprecated."

  No. You're using rsync because you're actively doing
  something _wrong_.

The deprecation sentiment was reinforced in 2008, with a
mention that cloning via rsync is broken (with no fix)[7].

Even the commit porting rsync over to C from shell (cd547b4)
lists it as deprecated! So between the 10 years of informal
warnings, and the fact that it has been severely broken
since 2007, it's probably safe to simply remove it without
further deprecation warnings.

[1] http://article.gmane.org/gmane.comp.version-control.git/285101
[2] http://article.gmane.org/gmane.comp.version-control.git/10093
[3] http://article.gmane.org/gmane.comp.version-control.git/17734
[4] http://article.gmane.org/gmane.comp.version-control.git/18911
[5] http://article.gmane.org/gmane.comp.version-control.git/5617
[6] http://article.gmane.org/gmane.comp.version-control.git/19354
[7] http://article.gmane.org/gmane.comp.version-control.git/103635

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitcore-tutorial.txt

@@ -710,7 +710,7 @@ files).
 Again, this can all be simplified with
 
 ----------------
-$ git clone rsync://rsync.kernel.org/pub/scm/git/git.git/ my-git
+$ git clone git://git.kernel.org/pub/scm/git/git.git/ my-git
 $ cd my-git
 $ git checkout
 ----------------
@@ -1011,20 +1011,6 @@ $ git fetch <remote-repository>
 One of the following transports can be used to name the
 repository to download from:
 
-Rsync::
-	`rsync://remote.machine/path/to/repo.git/`
-+
-Rsync transport is usable for both uploading and downloading,
-but is completely unaware of what git does, and can produce
-unexpected results when you download from the public repository
-while the repository owner is uploading into it via `rsync`
-transport.  Most notably, it could update the files under
-`refs/` which holds the object name of the topmost commits
-before uploading the files in `objects/` -- the downloader would
-obtain head commit object name while that object itself is still
-not available in the repository.  For this reason, it is
-considered deprecated.
-
 SSH::
 	`remote.machine:/path/to/repo.git/` or
 +
@@ -1430,7 +1416,7 @@ while, depending on how active your project is.
 
 When a repository is synchronized via `git push` and `git pull`
 objects packed in the source repository are usually stored
-unpacked in the destination, unless rsync transport is used.
+unpacked in the destination.
 While this allows you to use different packing strategies on
 both ends, it also means you may need to repack both
 repositories every once in a while.