prefer mercurial-2.8.2 for more robustness (i.e. the version used by the hosting service https://admin.phacility.com);

--- a/src/Doc/System/Phabricator.thy	Thu Jan 09 16:01:31 2020 +0100
+++ b/src/Doc/System/Phabricator.thy	Fri Jan 10 15:42:13 2020 +0100
@@ -419,9 +419,11 @@
   Option \<^verbatim>\<open>-U\<close> ensures a full update of system packages, before installing
   further packages required by Phabricator. This might require a reboot.
 
-  Option \<^verbatim>\<open>-M:\<close> installs a standard Mercurial release from source: this works
-  better than the package provided by Ubuntu 18.04. Alternatively, an explicit
-  file path or URL the source archive (\<^verbatim>\<open>.tar.gz\<close>) may be here. This option is
+  Option \<^verbatim>\<open>-M:\<close> installs a standard Mercurial release from source --- the one
+  that is used by the Phabricator hosting service
+  \<^url>\<open>https://admin.phacility.com\<close>. This avoids various problems with the
+  package provided by Ubuntu 18.04. Alternatively, an explicit file path or
+  URL the source archive (\<^verbatim>\<open>.tar.gz\<close>) may be given here. This option is
   recommended for production use, but it requires to \<^emph>\<open>uninstall\<close> existing
   Mercurial packages provided by the operating system.
 

--- a/src/Pure/Tools/phabricator.scala	Thu Jan 09 16:01:31 2020 +0100
+++ b/src/Pure/Tools/phabricator.scala	Fri Jan 10 15:42:13 2020 +0100
@@ -63,7 +63,7 @@
   val alternative_system_port = 222
   val default_server_port = 2222
 
-  val standard_mercurial_source = "https://www.mercurial-scm.org/release/mercurial-5.2.1.tar.gz"
+  val standard_mercurial_source = "https://www.mercurial-scm.org/release/mercurial-2.8.2.tar.gz"