git submodule ignores --git-dir

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