Correctly handle when ARGV is empty

If it's empty, in older bash versions bash will say "undefined variable"
when converting the array to a list of arguments. The `foo+...` part
says what to do in the edge case.

git-heatmap

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/bash
 
 # Build a commit frequency histogram
 
@@ -7,7 +7,7 @@ cnone="$(echo -ne '\033[0m')"
 
 set -euo pipefail
 
-argv=()
+ARGV=()
 while [[ $# -gt 0 ]]; do
   key="$1"
   case $key in
@@ -49,19 +49,21 @@ EOF
       exit
       ;;
     *)
-      argv+=("$1")
+      ARGV+=("$1")
       shift
       ;;
   esac
 done
 
 LIMIT=${LIMIT:-30}
-WIDTH=${WIDTH:-60}
-CHAR=${CHAR:-█}
 REVIEW_BASE=${REVIEW_BASE:-master}
+CHAR=${CHAR:-█}
+WIDTH=${WIDTH:-60}
 
 files() {
-  git log --name-status --pretty=format: -- "${argv[@]}" | cut -f 2-
+  # https://stackoverflow.com/questions/7577052/
+  git log --name-status --pretty=format: -- "${ARGV[@]+"${ARGV[@]}"}" | \
+    cut -f 2-
 }
 
 color_name() {