submodule: merge repo_read_gitmodules and gitmodules_config

Since 69aba5329 (submodule: add repo_read_gitmodules) there have been
two ways to load a repository's .gitmodules file:
'repo_read_gitmodules()' is used if you have a repository object you are
working with or 'gitmodules_config()' if you are implicitly working with
'the_repository'.  Merge the logic of these two functions to remove
duplicate code.

In addition, 'repo_read_gitmodules()' can segfault by passing in a NULL
pointer to 'git_config_from_file()' if a repository doesn't have a
worktree.  Instead check for the existence of a worktree before
attempting to load the .gitmodules file.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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