submodule-config: reuse config_from_gitmodules in repo_read_gitmodules

submodule-config: reuse config_from_gitmodules in repo_read_gitmodules

Reuse config_from_gitmodules in repo_read_gitmodules to remove some
duplication and also have a single point where the .gitmodules file is
read.

The change does not introduce any new behavior, the same gitmodules_cb
config callback is still used, which only deals with configuration
specific to submodules.

The check about the repo's worktree is removed from repo_read_gitmodules
because it's already performed in config_from_gitmodules.

The config_from_gitmodules function is moved up in the file —unchanged—
before its users to avoid a forward declaration.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Acked-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>