repack: document --unpack-unreachable option

This was added back in 7e52f56 (gc: do not explode objects
which will be immediately pruned, 2012-04-07), but not
documented at the time, since it was an internal detail
between git-gc and git-repack. However, as people with
complicated setups may want to effectively reimplement the
steps of git-gc themselves, it is nice for us to document
these interfaces.

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

Documentation/git-repack.txt

@@ -128,6 +128,12 @@ other objects in that pack they already have locally.
 	with `-b` or `repack.writeBitmaps`, as it ensures that the
 	bitmapped packfile has the necessary objects.
 
+--unpack-unreachable=<when>::
+	When loosening unreachable objects, do not bother loosening any
+	objects older than `<when>`. This can be used to optimize out
+	the write of any objects that would be immediately pruned by
+	a follow-up `git prune`.
+
 Configuration
 -------------