submodule-config: lazy-load a repository's .gitmodules file

submodule-config: lazy-load a repository's .gitmodules file

In order to use the submodule-config subsystem, callers first need to
initialize it by calling 'repo_read_gitmodules()' or
'gitmodules_config()' (which just redirects to
'repo_read_gitmodules()').  There are a couple of callers who need to
load an explicit revision of the repository's .gitmodules file (grep) or
need to modify the .gitmodules file so they would need to load it before
modify the file (checkout), but the majority of callers are simply
reading the .gitmodules file present in the working tree.  For the
common case it would be nice to avoid the boilerplate of initializing
the submodule-config system before using it, so instead let's perform
lazy-loading of the submodule-config system.

Remove the calls to reading the gitmodules file from ls-files to show
that lazy-loading the .gitmodules file works.

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