btrbk: enable --override option

btrbk

@@ -3046,7 +3046,7 @@ MAIN:
     'table|t'        => sub { $output_format = "table" },
     'format=s'       => \$output_format,
     'print-schedule' => \$print_schedule,
-    # 'override=s'    => \%config_override_opts,  # e.g. --override=incremental=no
+    'override=s'    => \%config_override_opts,  # e.g. --override=incremental=no
    ))
   {
     VERSION_MESSAGE();

doc/btrbk.1

@@ -112,6 +112,12 @@ Print detailed scheduler information on "run" and "dryrun"
 commands. Use the \fI\-\-format\fR command line option to switch
 between different output formats.
 .RE
+.PP
+\-\-override <config_option>=<value>
+.RS 4
+Override a configuration option <config_option> with <value>.
+Globally, for ALL contexts. Use with care!
+.RE
 .SH COMMANDS
 .PP
 .B run