Prevent msys2 conversion of "cmd /c" switch to a file path

Modern versions of msys2 have changed the treatment of "cmd /c" so that
the runtime will try to convert the switch to a native file path. This
patch adds a setting to inhibit that behaviour.

Discussion: https://postgr.es/m/3227042f-cfcc-745a-57dd-fb8c471f8ddf@2ndQuadrant.com

Backpatch to all live branches.

src/bin/pg_upgrade/test.sh

@@ -244,8 +244,11 @@ esac
 
 pg_ctl start -l "$logdir/postmaster2.log" -o "$POSTMASTER_OPTS" -w
 
+# In the commands below we inhibit msys2 from converting the "/c" switch
+# in "cmd /c" to a file system path.
+
 case $testhost in
-	MINGW*)	cmd /c analyze_new_cluster.bat ;;
+	MINGW*)	MSYS2_ARG_CONV_EXCL=/c cmd /c analyze_new_cluster.bat ;;
 	*)		sh ./analyze_new_cluster.sh ;;
 esac
 
@@ -258,7 +261,7 @@ if [ -n "$pg_dumpall2_status" ]; then
 fi
 
 case $testhost in
-	MINGW*)	cmd /c delete_old_cluster.bat ;;
+	MINGW*)	MSYS2_ARG_CONV_EXCL=/c cmd /c delete_old_cluster.bat ;;
 	*)	    sh ./delete_old_cluster.sh ;;
 esac