build: use nyc's merge command

a 'merge' command has recently been added to nyc, eliminating the need
for the istanbul-merge library.
PR-URL: #20760
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: Benjamin Coe

Makefile

@@ -165,8 +165,6 @@ coverage: coverage-test ## Run the tests and generate a coverage report.
 .PHONY: coverage-build
 coverage-build: all
 	mkdir -p node_modules
-if [ !-d node_modules/istanbul-merge ];then \
-$(NODE) ./deps/npm install istanbul-merge --no-save --no-package-lock;fi
 if [ !-d node_modules/nyc ];then \
 $(NODE) ./deps/npm install nyc --no-save --no-package-lock;fi
 if [ !-d gcovr ];then git clone -b 3.4 --depth=1 \
@@ -195,11 +193,11 @@ coverage-test: coverage-build
 	mv lib lib__
 	mv lib_ lib
 	mkdir -p coverage .cov_tmp
-$(NODE) ./node_modules/.bin/istanbul-merge --out \
- .cov_tmp/libcov.json'out/Release/.coverage/coverage-*.json'
+$(NODE) ./node_modules/.bin/nyc merge 'out/Release/.coverage'\
+ .cov_tmp/libcov.json
 	(cd lib && .$(NODE) ../node_modules/.bin/nyc report \
  --temp-directory "$(CURDIR)/.cov_tmp"\
- --report-dir "../coverage")
+ --report-dir "$(CURDIR)/coverage")
 	-(cd out &&"../gcovr/scripts/gcovr" --gcov-exclude='.*deps' \
  --gcov-exclude='.*usr' -v -r Release/obj.target \
  --html --html-detail -o ../coverage/cxxcoverage.html \