Fix / work around the bug where 'ssh -o ControlMaster=auto'
interacts badly with the svn+ssh tunnel scheme.  As I note in
the bug log for #413102, I don't really think the problem is
either svn's or openssh's fault, but the obvious way to avoid
it is to avoid accidentally becoming a ControlMaster.  In this
mode we will still use ssh connection sharing, but only if a
ControlMaster is already present.

diff --git a/subversion/subversion/libsvn_ra_svn/client.c b/subversion/subversion/libsvn_ra_svn/client.c
index dd60dbef9..afdd12a14 100644
--- a/subversion/subversion/libsvn_ra_svn/client.c
+++ b/subversion/subversion/libsvn_ra_svn/client.c
@@ -399,7 +399,7 @@ static svn_error_t *find_tunnel_agent(const char *tunnel,
        * versions have it too. If the user is using some other ssh
        * implementation that doesn't accept it, they can override it
        * in the [tunnels] section of the config. */
-      val = "$SVN_SSH ssh -q --";
+      val = "$SVN_SSH ssh -q -o ControlMaster=no --";
     }
 
   if (!val || !*val)
diff --git a/subversion/subversion/libsvn_subr/config_file.c b/subversion/subversion/libsvn_subr/config_file.c
index 9c0a83a17..ffb9524dc 100644
--- a/subversion/subversion/libsvn_subr/config_file.c
+++ b/subversion/subversion/libsvn_subr/config_file.c
@@ -1447,7 +1447,7 @@ svn_config_ensure(const char *config_dir, apr_pool_t *pool)
         "### passed to the tunnel agent as <user>@<hostname>.)  If the"      NL
         "### built-in ssh scheme were not predefined, it could be defined"   NL
         "### as:"                                                            NL
-        "# ssh = $SVN_SSH ssh -q --"                                         NL
+        "# ssh = $SVN_SSH ssh -q -o ControlMaster=no --"                     NL
         "### If you wanted to define a new 'rsh' scheme, to be used with"    NL
         "### 'svn+rsh:' URLs, you could do so as follows:"                   NL
         "# rsh = rsh --"                                                     NL