update: passing --force to git fetch

Per document:
> -f, --force
>    When git fetch is used with <rbranch>:<lbranch> refspec, it refuses
>    to update the local branch <lbranch> unless the remote branch
>    <rbranch> it fetches is a descendant of <lbranch>. This option
>    overrides that check.

Library/Homebrew/cmd/update.sh

@@ -349,7 +349,7 @@ EOS
         [[ "$UPSTREAM_SHA_HTTP_CODE" = "304" ]] && exit
       fi
 
-      git fetch "${QUIET_ARGS[@]}" origin \
+      git fetch --force "${QUIET_ARGS[@]}" origin \
         "refs/heads/$UPSTREAM_BRANCH:refs/remotes/origin/$UPSTREAM_BRANCH" || \
           odie "Fetching $DIR failed!"
     ) &