let pack-objects do the writing of unreachable objects as loose objects

Commit ccc1297226 changed the behavior
of 'git repack -A' so unreachable objects are stored as loose objects.
However it did so in a naive and inn efficient way by making packs
about to be deleted inaccessible and feeding their content through
'git unpack-objects'.  While this works, there are major flaws with
this approach:

- It is unacceptably sloooooooooooooow.

  In the Linux kernel repository with no actual unreachable objects,
  doing 'git repack -A -d' before:

	real    2m33.220s
	user    2m21.675s
	sys     0m3.510s

  And with this change:

	real    0m36.849s
	user    0m24.365s
	sys     0m1.950s

  For reference, here's the timing for 'git repack -a -d':

	real    0m35.816s
	user    0m22.571s
	sys     0m2.011s

  This is explained by the fact that 'git unpack-objects' was used to
  unpack _every_ objects even if (almost) 100% of them were thrown away.

- There is a black out period.

  Between the removal of the .idx file for the redundant pack and the
  completion of its unpacking, the unreachable objects become completely
  unaccessible.  This is not a big issue as we're talking about unreachable
  objects, but some consistency is always good.

- There is no way to easily set a sensible mtime for the newly created
  unreachable loose objects.

So, while having a command called "pack-objects" to perform object
unpacking looks really odd, this is probably the best compromize to be
able to solve the above issues in an efficient way.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-repack.sh

@@ -8,7 +8,7 @@ OPTIONS_SPEC="\
 git-repack [options]
 --
 a               pack everything in a single pack
-A               same as -a, and keep unreachable objects too
+A               same as -a, and turn unreachable objects loose
 d               remove redundant packs, and run git-prune-packed
 f               pass --no-reuse-delta to git-pack-objects
 q,quiet         be quiet
@@ -22,7 +22,7 @@ max-pack-size=  maximum size of each packfile
 SUBDIRECTORY_OK='Yes'
 . git-sh-setup
 
-no_update_info= all_into_one= remove_redundant= keep_unreachable=
+no_update_info= all_into_one= remove_redundant= unpack_unreachable=
 local= quiet= no_reuse= extra=
 while test $# != 0
 do
@@ -30,7 +30,7 @@ do
 	-n)	no_update_info=t ;;
 	-a)	all_into_one=t ;;
 	-A)	all_into_one=t
-		keep_unreachable=t ;;
+		unpack_unreachable=--unpack-unreachable ;;
 	-d)	remove_redundant=t ;;
 	-q)	quiet=-q ;;
 	-f)	no_reuse=--no-reuse-object ;;
@@ -78,6 +78,9 @@ case ",$all_into_one," in
 	if test -z "$args"
 	then
 		args='--unpacked --incremental'
+	elif test -n "$unpack_unreachable"
+	then
+		args="$args $unpack_unreachable"
 	fi
 	;;
 esac
@@ -127,18 +130,7 @@ then
 		  do
 			case " $fullbases " in
 			*" $e "*) ;;
-			*)
-				rm -f "$e.idx" "$e.keep"
-				if test -n "$keep_unreachable" &&
-				   test -f "$e.pack"
-				then
-					git unpack-objects < "$e.pack" || {
-						echo >&2 "Failed unpacking unreachable objects from redundant pack file $e.pack"
-						exit 1
-					}
-				fi
-				rm -f "$e.pack"
-			;;
+			*)	rm -f "$e.pack" "$e.idx" "$e.keep" ;;
 			esac
 		  done
 		)