macports: skip OS check for migrate/selfupdate

When running migration and/or selfupdate, we specifically do not want to
check for outdated OS versions.

src/macports1.0/macports.tcl

@@ -692,7 +692,8 @@ proc mportinit {{up_ui_options {}} {up_options {}} {up_variations {}}} {
         macosx_version
 
     # Check that the current platform is the one we were configured for, otherwise need to do migration
-    if {[migrate::needs_migration]} {
+    set skip_migration_check [expr {[info exists macports::global_options(ports_no_migration_check)] && $macports::global_options(ports_no_migration_check)}]
+    if {!$skip_migration_check && [migrate::needs_migration]} {
         ui_error "Current platform \"$os_platform $os_major\" does not match expected platform \"$macports::autoconf::os_platform $macports::autoconf::os_major\""
         ui_error "Please run 'sudo port migrate' or follow the migration instructions: https://trac.macports.org/wiki/Migration"
         return -code error "OS platform mismatch"

src/port/port.tcl

@@ -5764,7 +5764,11 @@ set remaining_args [lrange $cmd_argv $cmd_argn end]
 # shell mode
 if { [llength $remaining_args] == 0 && ![info exists ui_options(ports_commandfiles)] } {
     lappend ui_options(ports_commandfiles) -
-} elseif {[lookahead] eq "selfupdate" || [lookahead] eq "sync"} {
+} elseif {[lookahead] in {"selfupdate" "migrate"}} {
+    # tell mportinit not to tell the user they should selfupdate and skip the migration check
+    set ui_options(ports_no_old_index_warning) 1
+    set global_options(ports_no_migration_check) 1
+} elseif {[lookahead] eq "sync"} {
     # tell mportinit not to tell the user they should selfupdate
     set ui_options(ports_no_old_index_warning) 1
 }