pack-redundant: document deprecation

Running the command itself has generated a warning for several versions,
which has recently been upgraded to an error. Let's also make sure the
documentation mentions what is going on. This also gives us a good spot
to explain the reasoning and recommend alternatives.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

Documentation/git-pack-redundant.txt

@@ -11,6 +11,20 @@ SYNOPSIS
 [verse]
 'git pack-redundant' [--verbose] [--alt-odb] (--all | <pack-filename>...)
 
+WARNING
+-------
+`git pack-redundant` has been deprecated and is scheduled for removal in
+a future version of Git. Because it can only remove entire duplicate
+packs and not individual duplicate objects, it is generally not a useful
+tool for reducing repository size. You are better off using `git gc` to
+do so, which will put objects into a new pack, removing duplicates.
+
+Running `pack-redundant` without the `--i-still-use-this` flag will fail
+in this release. If you believe you have a use case for which
+`pack-redundant` is better suited and oppose this removal, please
+contact the Git mailing list at [email protected]. More information
+about the list is available at https://git-scm.com/community.
+
 DESCRIPTION
 -----------
 This program computes which packs in your repository