Check if pack directory exists prior to descending into it

This fixes the following warning:

git-repack: line 42: cd: .git/objects/pack: No such file or directory

This happens only, when git-repack -a is run without any packs in the
repository.

Signed-off-by: Matthias Kestenholz <matthias@spinlock.ch>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-repack.sh

@@ -38,7 +38,7 @@ case ",$all_into_one," in
 	pack_objects=
 
 	# Redundancy check in all-into-one case is trivial.
-	existing=`cd "$PACKDIR" && \
+	existing=`test -d "$PACKDIR" && cd "$PACKDIR" && \
 	    find . -type f \( -name '*.pack' -o -name '*.idx' \) -print`
 	;;
 esac