fetch: avoid reading submodule config until needed

In "fetch", there are two parameters submodule_fetch_jobs_config and
recurse_submodules that can be set in a variety of ways: through
.gitmodules, through .git/config, and through the command line.
Currently "fetch" handles this by first reading .gitmodules, then
reading .git/config (allowing it to overwrite existing values), then
reading the command line (allowing it to overwrite existing values).

Notice that we can avoid reading .gitmodules if .git/config and/or the
command line already provides us with what we need. In addition, if
recurse_submodules is found to be "no", we do not need the value of
submodule_fetch_jobs_config.

Avoiding reading .gitmodules is especially important when we use "git
fetch" to perform lazy fetches in a partial clone because the
.gitmodules file itself might need to be lazy fetched (and otherwise
causing an infinite loop).

In light of all this, avoid reading .gitmodules until necessary. When
reading it, we may only need one of the two parameters it provides, so
teach fetch_config_from_gitmodules() to support NULL arguments. With
this patch, users (including Git itself when invoking "git fetch" to
lazy-fetch) will be able to guarantee avoiding reading .gitmodules by
passing --recurse-submodules=no.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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