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

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
 	;;