gc: merge prune and gc into single step

pass --prune=all to gc to ensure that we properly cleanup unpacked and
packed objects and don't have the weird bloating effect after a first
pass, e.g.:

  $ asp disk-usage
  ==> Using 640K on disk.

  $ asp gc
  Nothing new to pack.

  $ asp disk-usage
  ==> Using 8.4M on disk.

  $ asp gc
  Nothing new to pack.

  $ asp disk-usage
  ==> Using 328K on disk.

1 files changed, 1 insertions, 2 deletions

diff --git a/asp.in b/asp.in
index d22f81e..2e6805e 100644
--- a/asp.in
+++ b/asp.in
@@ -176,8 +176,7 @@ difflog() {
 }
 
 gc() {
-  git prune
-  git gc
+  git gc --prune=all
 }
 
 untrack() {