Use Git submodule tracking feature

This way we can point submodules to different branches.

There are two side-effects to this:

- Git 1.8.2 becomes the minimal required version now
  to support this feature.

- Not sure how doing local changes in submodules followed
  by `make update` will behave. We don't use explicit rebase
  now.

  Perhaps this is not so bad, since it was already quite
  dangerous thing to do.

.gitmodules

@@ -1,16 +1,20 @@
 [submodule "release/scripts/addons"]
 	path = release/scripts/addons
 	url = ../blender-addons.git
+	branch = blender2.8
 	ignore = all
 [submodule "release/scripts/addons_contrib"]
 	path = release/scripts/addons_contrib
 	url = ../blender-addons-contrib.git
+	branch = master
 	ignore = all
 [submodule "release/datafiles/locale"]
 	path = release/datafiles/locale
 	url = ../blender-translations.git
+	branch = master
 	ignore = all
 [submodule "source/tools"]
 	path = source/tools
 	url = ../blender-dev-tools.git
+	branch = master
 	ignore = all

GNUmakefile

@@ -402,7 +402,7 @@ update: .FORCE
 		svn update ../lib/* ; \
 	fi
 	git pull --rebase
-	git submodule foreach git pull --rebase origin master
+	git submodule update --remote
 
 
 # -----------------------------------------------------------------------------

make.bat

@@ -90,7 +90,7 @@ if NOT "%1" == "" (
 	) else if "%1" == "update" (
 		svn up ../lib/*
 		git pull --rebase
-		git submodule foreach git pull --rebase origin master
+		git submodule update --remote
 		goto EOF
 	) else if "%1" == "clean" (
 		set MUST_CLEAN=1