Update "Verifying that updates still work" (#1290)

Convert the instructions into a single script that anyone can copy-paste to a terminal.
Add `make destroy-db` to the initialization part.

CONTRIBUTING.md

@@ -46,30 +46,43 @@ When you are making PR that adds new spatial features to OpenMapTiles schema, pl
 
 It is recommended that you create a [unit test](TESTING.md) when modifying the behavior of the SQL layer.  This will ensure that your changes are working as expected when importing or updating OSM data into an OpenMapTiles database.
 
-To run the unit tests, run `make clean destroy-db && make test-sql`
-
 # Verifying that updates still work
 
-When testing a PR, you shoud also verify that the update process completes without error.  The following procedure will run the update process.
+When testing a PR, you should also verify that the update process completes without an error. Please modify, if necessary, and run the script below.
 
-1. Change .env to set `DIFF_MODE=true`.  In addition, if you're testing changes that impact zooms higher than the default zoom of 7, you should also change `MAX_ZOOM` to be up to 14.
-2. Set the test area to the appropriate geofabrik extract, for example:
+**Note:**
+
+The verification requires the script to append temporary changes to the `.env` file. Please restore the original version from git using `git checkout .env` or remove these changes before submitting a PR.
 
 ```
+(
+set -e
+
+cat >> .env << EOM
+
+# temporary changes for verifying that updates still work
+# Ensure DIFF_MODE is active
+DIFF_MODE=true
+# Ensure all zoom levels are tested
+MAX_ZOOM=14
+EOM
+
+# Set the test area to the appropriate geofabrik extract
 export area=north-america/us/indiana
-```
 
-3. Build 1-month-old tiles:
-
-```
+# Build 1-month-old tiles
 rm -fr data build cache
+make destroy-db
 make download-geofabrik area=$area
-docker-compose run --rm --user=$(id -u):$(id -g) openmaptiles-tools sh -c "wget -O data/$area.osm.pbf http://download.geofabrik.de/$area-$(date --date="$(date +%Y-%m-15) -1 month" +'%y%m01').osm.pbf"
+docker-compose run --rm --user=$(id -u):$(id -g) openmaptiles-tools sh -c "wget -nv -O data/$area.osm.pbf http://download.geofabrik.de/$area-$(date --date="$(date +%Y-%m-15) -1 month" +'%y%m01').osm.pbf"
 ./quickstart.sh $area
-```
-4. Update with the changes since then:
-```
+cat << EOM
+
+# Update with the changes since a month+ ago
+
+EOM
 docker-compose run --rm --user=$(id -u):$(id -g) openmaptiles-tools sh -c "osmupdate --base-url=$(sed -n 's/ *\"replication_url\": //p' data/$area.repl.json) data/$area.osm.pbf data/changes.osc.gz"
 make import-diff
 make generate-tiles-pg
+) < /dev/null
 ```