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Rename git_submodule_update function

This commit renames the existing `git_submodule_update` function to
`git_submodule_update_strategy`. The`git_submodule_update` function now
refers to functionality similar to `git submodule update`, while
`git_submodule_update_strategy` is used to get the configured value of
submodule.<name>.update.
Update CHANGELOG for breaking API change