Update UPDATE.md including Makefile improvements (#1408)

- Add `make import-diff` and `make generate-changed-tiles`
- Update `UPDATE.md` accordingly
- Other updates and clarifications in `UPDATE.md`

Makefile

@@ -186,6 +186,7 @@ Hints for developers:
   make generate-qa                     # statistics for a given layer's field
   make generate-tiles-pg               # generate vector tiles based on .env settings using PostGIS ST_MVT()
   make generate-tiles                  # generate vector tiles based on .env settings using Mapnik (obsolete)
+  make generate-changed-tiles          # Generate tiles changed by import-diff
   make test-sql                        # run unit tests on the OpenMapTiles SQL schema
   cat  .env                            # list PG database and MIN_ZOOM and MAX_ZOOM information
   cat  quickstart.log                  # transcript of the last ./quickstart.sh run
@@ -200,6 +201,7 @@ Hints for downloading & importing data:
   make download-bbbike area=Amsterdam  # download OSM data from bbbike.org       and create config file
   make import-data                     # Import data from OpenStreetMapData, Natural Earth and OSM Lake Labels.
   make import-osm                      # Import OSM data with the mapping rules from build/mapping.yaml
+  make import-diff                     # Import OSM updates from data/changes.osc.gz
   make import-wikidata                 # Import labels from Wikidata
   make import-sql                      # Import layers (run this after modifying layer SQL)
 
@@ -397,7 +399,7 @@ import-osm: all start-db-nowait
 	$(DOCKER_COMPOSE) $(DC_CONFIG_CACHE) run $(DC_OPTS_CACHE) openmaptiles-tools sh -c 'pgwait && import-osm $(PBF_FILE)'
 
 .PHONY: start-update-osm
-start-update-osm: all start-db
+start-update-osm: start-db
 	@$(assert_area_is_given)
 	$(DOCKER_COMPOSE) $(DC_CONFIG_CACHE) up -d update-osm
 
@@ -406,7 +408,7 @@ stop-update-osm:
 	$(DOCKER_COMPOSE) stop update-osm
 
 .PHONY: import-diff
-import-diff: all start-db-nowait
+import-diff: start-db-nowait
 	@$(assert_area_is_given)
 	$(DOCKER_COMPOSE) $(DC_CONFIG_CACHE) run $(DC_OPTS_CACHE) openmaptiles-tools sh -c 'pgwait && import-diff'
 
@@ -440,6 +442,21 @@ generate-tiles-pg: all start-db
 	$(DOCKER_COMPOSE) run $(DC_OPTS) openmaptiles-tools \
 			mbtiles-tools meta-generate "$(MBTILES_LOCAL_FILE)" $(TILESET_FILE) --auto-minmax --show-ranges
 
+.PHONY: data/tiles.txt
+data/tiles.txt:
+	find ./data -name "*.tiles" -exec cat {} \; -exec rm {} \; | \
+	  $(DOCKER_COMPOSE) run $(DC_OPTS) openmaptiles-tools \
+	    tile_multiplier $(MIN_ZOOM) $(MAX_ZOOM) >> data/tiles.txt
+
+.PHONY: generate-changed-tiles
+generate-changed-tiles: data/tiles.txt
+	# Re-generating updated tiles, if needed
+	if [ -s data/tiles.txt ] ; then \
+	  $(DOCKER_COMPOSE) $(DC_CONFIG_CACHE) run $(DC_OPTS_CACHE) openmaptiles-tools refresh-views; \
+	  $(DOCKER_COMPOSE) run $(DC_OPTS) -e LIST_FILE=data/tiles.txt openmaptiles-tools generate-tiles; \
+	  rm data/tiles.txt; \
+	fi
+
 .PHONY: start-tileserver
 start-tileserver: init-dirs
 	@echo " "