rehabilitate 'git index-pack' inside the object store

Before commit d0b92a3f6e it was possible to run 'git index-pack'
directly in the .git/objects/pack/ directory.  Restore that ability.

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

t/t5302-pack-index.sh

@@ -177,4 +177,14 @@ test_expect_success \
        ".git/objects/pack/pack-${pack1}.pack" 2>&1) &&
      echo "$err" | grep "CRC mismatch"'
 
+test_expect_success 'running index-pack in the object store' '
+    rm -f .git/objects/pack/* &&
+    cp test-1-${pack1}.pack .git/objects/pack/pack-${pack1}.pack &&
+    (
+	cd .git/objects/pack
+	git index-pack pack-${pack1}.pack
+    ) &&
+    test -f .git/objects/pack/pack-${pack1}.idx
+'
+
 test_done