submodule: explicit local branch creation in module_clone

The previous code only checked out branches in cmd_add. This commit moves the branch-checkout logic into module_clone, where it can be shared by cmd_add and cmd_update. I also update the initial checkout command to use 'reset' to preserve branches setup during module_clone. With this change, folks cloning submodules for the first time via: $ git submodule update ... will get a local branch instead of a detached HEAD, unless they are using the default checkout-mode updates. This is a change from the previous situation where cmd_update always used checkout-mode logic (regardless of the requested update mode) for updates that triggered an initial clone, which always resulted in a detached HEAD. This commit does not change the logic for updates after the initial clone, which will continue to create detached HEADs for checkout-mode updates, and integrate remote work with the local HEAD (detached or not) in other modes. The motivation for the change is that developers doing local work inside the submodule are likely to select a non-checkout-mode for updates so their local work is integrated with upstream work. Developers who are not doing local submodule work stick with checkout-mode updates so any apparently local work is blown away during updates. For example, if upstream rolls back the remote branch or gitlinked commit to an earlier version, the checkout-mode developer wants their old submodule checkout to be rolled back as well, instead of getting a no-op merge/rebase with the rolled-back reference. By using the update mode to distinguish submodule developers from black-box submodule consumers, we can setup local branches for the developers who will want local branches, and stick with detached HEADs for the developers that don't care. Testing ======= In t7406, just-cloned checkouts now update to the gitlinked hash with 'reset', to preserve the local branch for situations where we're not on a detached HEAD. I also added explicit tests to t7406 for HEAD attachement after cloning updates, showing that it depends on their update mode: * Checkout-mode updates get detached HEADs * Everyone else gets a local branch, matching the configured submodule.<name>.branch and defaulting to master. The 'initial-setup' tag makes it easy to reset the superproject to a known state, as several earlier tests commit to submodules and commit the changed gitlinks to the superproject, but don't push the new submodule commits to the upstream subprojects. This makes it impossible to checkout the current super master, because it references submodule commits that don't exist in the upstream subprojects. For a specific example, see the tests that currently generate the 'two_new_submodule_commits' commits. Documentation ============= I updated the docs to describe the 'submodule update' modes in detail. The old documentation did not distinguish between cloning and non-cloning updates and lacked clarity on which operations would lead to detached HEADs, and which would not. The new documentation addresses these issues while updating the docs to reflect the changes introduced by this commit's explicit local branch creation in module_clone. I also add '--checkout' to the usage summary and group the update-mode options into a single set. Signed-off-by: W. Trevor King <wking@tremily.us> Signed-off-by: Junio C Hamano <gitster@pobox.com>