git-repack.txt: Clarify implications of -a for dumb protocols

The current text makes some users feel uneasy, worrying whether
'-a' could lead to corrupt repositories. Clarify that '-a'
may lead to performance issues only for dumb protocols.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Helped-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-repack.txt

@@ -31,11 +31,14 @@ OPTIONS
 	Instead of incrementally packing the unpacked objects,
 	pack everything referenced into a single pack.
 	Especially useful when packing a repository that is used
-	for private development and there is no need to worry
-	about people fetching via dumb protocols from it.  Use
+	for private development. Use
 	with '-d'.  This will clean up the objects that `git prune`
 	leaves behind, but `git fsck --full` shows as
 	dangling.
++
+Note that users fetching over dumb protocols will have to fetch the
+whole new pack in order to get any contained object, no matter how many
+other objects in that pack they already have locally.
 
 -A::
 	Same as `-a`, unless '-d' is used.  Then any unreachable