git submodule ignores --git-dir

[Filip Gospodinov <filip.gospodinov@monetas.net>]

Hi!

I use the `--git-dir` flag in some scripts such that I don't need to `cd` back
and forth. Recently, I've discovered that `--git-dir` does not seem to work
correctly for `git submodule`. Here is a short snippet to reproduce that behavior:

mkdir repo1 subm
(cd subm; git init; git commit -m 1 --allow-empty)
(cd repo1; git init; git submodule add ../subm subm; git commit -m "add subm")
git clone repo1 repo2
git --git-dir=$PWD/repo2/.git submodule update --init


which errors with the following output:

No submodule mapping found in .gitmodules for path 'subm'

But this works:
cd repo2; git --git-dir=$PWD/.git submodule update --init


I know that for this particular use case I can just use `git clone --recursive`
and that other use cases can be worked around by using `cd`. Still, I wonder if
the behavior I discovered is a bug or if it's expected.

git --version
git version 2.5.1

Thanks you!
Filip