Index: subversion/libsvn_client/diff.c
===================================================================
--- subversion/libsvn_client/diff.c	(revision 29979)
+++ subversion/libsvn_client/diff.c	(working copy)
@@ -566,10 +566,12 @@
                                        SVN_CONFIG_CATEGORY_CONFIG,
                                        APR_HASH_KEY_STRING);
       svn_config_get(cfg, &diff_cmd, SVN_CONFIG_SECTION_HELPERS,
-                     SVN_CONFIG_OPTION_DIFF_CMD, NULL);
+                     SVN_CONFIG_OPTION_DIFF_CMD, "");
     }
 
-  if (diff_cmd)
+  /* "" means use internal diff so that an option in a config file can be
+     overridden by a command-line setting. */
+  if (strlen(diff_cmd))
     {
       /* Print out the diff header. */
       SVN_ERR(svn_stream_printf_from_utf8
Index: subversion/svn/main.c
===================================================================
--- subversion/svn/main.c	(revision 29979)
+++ subversion/svn/main.c	(working copy)
@@ -211,7 +211,8 @@
   {"ignore-externals", opt_ignore_externals, 0,
                     N_("ignore externals definitions")},
 #ifndef AS400
-  {"diff-cmd",      opt_diff_cmd, 1, N_("use ARG as diff command")},
+  {"diff-cmd",      opt_diff_cmd, 1, N_("use ARG as diff command ('' for "
+					"built-in diff)")},
   {"diff3-cmd",     opt_merge_cmd, 1, N_("use ARG as merge command")},
   {"editor-cmd",    opt_editor_cmd, 1, N_("use ARG as external editor")},
 #endif