submodule update: add --remote for submodule's upstream changes

The current `update` command incorporates the superproject's gitlinked SHA-1 ($sha1) into the submodule HEAD ($subsha1). Depending on the options you use, it may checkout $sha1, rebase the $subsha1 onto $sha1, or merge $sha1 into $subsha1. This helps you keep up with changes in the upstream superproject. However, it's also useful to stay up to date with changes in the upstream subproject. Previous workflows for incorporating such changes include the ungainly: $ git submodule foreach 'git checkout $(git config --file $toplevel/.gitmodules submodule.$name.branch) && git pull' With this patch, all of the useful functionality for incorporating superproject changes can be reused to incorporate upstream subproject updates. When you specify --remote, the target $sha1 is replaced with a $sha1 of the submodule's origin/master tracking branch. If you want to merge a different tracking branch, you can configure the `submodule.<name>.branch` option in `.gitmodules`. You can override the `.gitmodules` configuration setting for a particular superproject by configuring the option in that superproject's default configuration (using the usual configuration hierarchy, e.g. `.git/config`, `~/.gitconfig`, etc.). Previous use of submodule.<name>.branch ======================================= Because we're adding a new configuration option, it's a good idea to check if anyone else is already using the option. The foreach-pull example above was described by Ævar in commit f030c96d8643fa0a1a9b2bd9c2f36a77721fb61f Author: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Date: Fri May 21 16:10:10 2010 +0000 git-submodule foreach: Add $toplevel variable Gerrit uses the same interpretation for the setting, but because Gerrit has direct access to the subproject repositories, it updates the superproject repositories automatically when a subproject changes. Gerrit also accepts the special value '.', which it expands into the superproject's branch name. Although the --remote functionality is using `submodule.<name>.branch` slightly differently, the effect is the same. The foreach-pull example uses the option to record the name of the local branch to checkout before pulls. The tracking branch to be pulled is recorded in `.git/modules/<name>/config`, which was initialized by the module clone during `submodule add` or `submodule init`. Because the branch name stored in `submodule.<name>.branch` was likely the same as the branch name used during the initial `submodule add`, the same branch will be pulled in each workflow. Implementation details ====================== In order to ensure a current tracking branch state, `update --remote` fetches the submodule's remote repository before calculating the SHA-1. However, I didn't change the logic guarding the existing fetch: if test -z "$nofetch" then # Run fetch only if $sha1 isn't present or it # is not reachable from a ref. (clear_local_git_env; cd "$path" && ( (rev=$(git rev-list -n 1 $sha1 --not --all 2>/dev/null) && test -z "$rev") || git-fetch)) || die "$(eval_gettext "Unable to fetch in submodule path '\$path'")" fi There will not be a double-fetch, because the new $sha1 determined after the `--remote` triggered fetch should always exist in the repository. If it doesn't, it's because some racy process removed it from the submodule's repository and we *should* be re-fetching. Signed-off-by: W. Trevor King <wking@tremily.us> Signed-off-by: Junio C Hamano <gitster@pobox.com>