buildrpm: fix embarassing bug in shell variable expansions `$distance_$node` looks up and concatenates `$distance_` and `$node`. `$distance_` is empty, so we were getting the node without the distance. Using the curly braces makes our intent explicit and produces better-versioned RPMS. Differential Revision: https://phab.mercurial-scm.org/D4493

--- a/contrib/packaging/buildrpm	Wed Sep 05 21:49:44 2018 +0900
+++ b/contrib/packaging/buildrpm	Wed Sep 05 16:29:31 2018 -0400
@@ -59,7 +59,7 @@
 fi
 
 if [ -n "$distance" ] ; then
-    release=$release+$distance_$node
+    release=$release+${distance}_${node}
 fi
 
 if [ "$PYTHONVER" ]; then