help: add a missing blank line to unhide `revlog-compression` stable

help: add a missing blank line to unhide `revlog-compression`

The help was output, but it was elided with "Enabled by default" from the
previous item.

--- a/mercurial/help/config.txt	Fri Jun 21 03:50:40 2019 +0200
+++ b/mercurial/help/config.txt	Sat Jun 22 23:04:52 2019 -0400
@@ -866,6 +866,7 @@
     Repositories with this on-disk format require Mercurial version 4.7
 
     Enabled by default.
+
 ``revlog-compression``
     Compression algorithm used by revlog. Supported value are `zlib` and `zstd`.
     The `zlib` engine is the historical default of Mercurial. `zstd` is a newer

--- a/tests/test-help.t	Fri Jun 21 03:50:40 2019 +0200
+++ b/tests/test-help.t	Sat Jun 22 23:04:52 2019 -0400
@@ -1509,6 +1509,8 @@
   
       "sparse-revlog"
   
+      "revlog-compression"
+  
       "profiling"
       -----------