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

contrib/packaging/buildrpm

@@ -59,7 +59,7 b' else'
 fi
 
 if [ -n "$distance" ] ; then
-    release=$release+$distance_$node
+    release=$release+${distance}_${node}
 fi
 
 if [ "$PYTHONVER" ]; then