git-fetch-script: fix http:// breakage

We were trying to fetch using the merge-head name rather than the
merge-head SHA1 that we just got.

Now, http:// is broken anyway right now for packing, but this should
make it work for nonpacked repositories again.

git-fetch-script

@@ -12,7 +12,7 @@ case "$merge_repo" in
 http://*)
 	head=$(wget -q -O - "$merge_repo/$merge_head") || exit 1
 	echo Fetching "$merge_head" using http
-	git-http-pull -v -a "$merge_head" "$merge_repo/"
+	git-http-pull -v -a "$head" "$merge_repo/"
 	;;
 rsync://*)
 	rsync -L "$merge_repo/$merge_head" "$TMP_HEAD" || exit 1