add: warn when adding an embedded repository

It's an easy mistake to add a repository inside another
repository, like:

  git clone $url
  git add .

The resulting entry is a gitlink, but there's no matching
.gitmodules entry. Trying to use "submodule init" (or clone
with --recursive) doesn't do anything useful. Prior to
v2.13, such an entry caused git-submodule to barf entirely.
In v2.13, the entry is considered "inactive" and quietly
ignored. Either way, no clone of your repository can do
anything useful with the gitlink without the user manually
adding the submodule config.

In most cases, the user probably meant to either add a real
submodule, or they forgot to put the embedded repository in
their .gitignore file.

Let's issue a warning when we see this case. There are a few
things to note:

  - the warning will go in the git-add porcelain; anybody
    wanting to do low-level manipulation of the index is
    welcome to create whatever funny states they want.

  - we detect the case by looking for a newly added gitlink;
    updates via "git add submodule" are perfectly reasonable,
    and this avoids us having to investigate .gitmodules
    entirely

  - there's a command-line option to suppress the warning.
    This is needed for git-submodule itself (which adds the
    entry before adding any submodule config), but also
    provides a mechanism for other scripts doing
    submodule-like things.

We could make this a hard error instead of a warning.
However, we do add lots of sub-repos in our test suite. It's
not _wrong_ to do so. It just creates a state where users
may be surprised. Pointing them in the right direction with
a gentle hint is probably the best option.

There is a config knob that can disable the (long) hint. But
I intentionally omitted a config knob to disable the warning
entirely. Whether the warning is sensible or not is
generally about context, not about the user's preferences.
If there's a tool or workflow that adds gitlinks without
matching .gitmodules, it should probably be taught about the
new command-line option, rather than blanket-disabling the
warning.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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