+--
+Update the registered submodules to match what the superproject
+expects by cloning missing submodules and updating the working tree of
+the submodules. The "updating" can be done in several ways depending
+on command line options and the value of `submodule.<name>.update`
+configuration variable. Supported update procedures are:
+
+ checkout;; the commit recorded in the superproject will be
+ checked out in the submodule on a detached HEAD. This is
+ done when `--checkout` option is given, or no option is
+ given, and `submodule.<name>.update` is unset, or if it is
+ set to 'checkout'.
++
+If `--force` is specified, the submodule will be checked out (using
+`git checkout --force` if appropriate), even if the commit specified
+in the index of the containing repository already matches the commit
+checked out in the submodule.
+
+ rebase;; the current branch of the submodule will be rebased
+ onto the commit recorded in the superproject. This is done
+ when `--rebase` option is given, or no option is given, and
+ `submodule.<name>.update` is set to 'rebase'.
+
+ merge;; the commit recorded in the superproject will be merged
+ into the current branch in the submodule. This is done
+ when `--merge` option is given, or no option is given, and
+ `submodule.<name>.update` is set to 'merge'.
+
+ custom command;; arbitrary shell command that takes a single
+ argument (the sha1 of the commit recorded in the
+ superproject) is executed. This is done when no option is
+ given, and `submodule.<name>.update` has the form of
+ '!command'.
+
+When no option is given and `submodule.<name>.update` is set to 'none',
+the submodule is not updated.
+