Merge branch 'sg/osx-force-gcc-9'

TravisCI update.

* sg/osx-force-gcc-9:
  ci: build Git with GCC 9 in the 'osx-gcc' build job

Author: gitster

ci/install-dependencies.sh

@@ -49,8 +49,10 @@ osx-clang|osx-gcc)
 	brew install caskroom/cask/perforce
 	case "$jobname" in
 	osx-gcc)
-		brew link gcc ||
-		brew link gcc@8
+		brew install gcc@9
+		# Just in case the image is updated to contain gcc@9
+		# pre-installed but not linked.
+		brew link gcc@9
 		;;
 	esac
 	;;

ci/lib.sh

@@ -131,7 +131,6 @@ then
 		echo "$SYSTEM_TASKDEFINITIONSURI$SYSTEM_TEAMPROJECT/_build/results?buildId=$1"
 	}
 
-	BREW_INSTALL_PACKAGES=gcc@8
 	export GIT_PROVE_OPTS="--timer --jobs 10 --state=failed,slow,save"
 	export GIT_TEST_OPTS="--verbose-log -x --write-junit-xml"
 	MAKEFLAGS="$MAKEFLAGS --jobs=10"
@@ -182,7 +181,7 @@ linux-clang|linux-gcc)
 osx-clang|osx-gcc)
 	if [ "$jobname" = osx-gcc ]
 	then
-		export CC=gcc-8
+		export CC=gcc-9
 	fi
 
 	# t9810 occasionally fails on Travis CI OS X