Add support for exporting only init'ed submodules

[akien-mga]

My use case involves exporting a Git repository with only a subset of its submodules initialized and updated, as shown here:

$ git submodule 
 f60be494a43a49fd7ff61603e132c1e24c160d88 assets/lang (f60be49)
-7751cf73f5c06f1be21f5f31c3e2d9a7bacd3a93 dx9sdk
 be7425ef70231ab82930331959ab487d605d0482 ext/SPIRV-Cross (2018-08-07~7)
 9efe3367284d2d1eeb14fc302a2c12c36e3e255e ext/armips (v0.9-128-g9efe336)
 3d3ae7129d17643bc706da0a2eea85aafd10ab3a ext/discord-rpc (v3.1.0-20-g3d3ae71)
 29619b2312f7bc862221749f3f4d37c3e6a0dee2 ext/glslang (2.3-2139-g29619b23)
-73063f5002612c6bf64fe24f851cd5cc0d83eef9 ext/rapidjson
-7472c903ec771c00af9925097e8d37075e9a379f ffmpeg
-d02ba7407050f445edf9e908374ad4bf3b2f237b pspautotests

It used to work fine in the past with git-archive-all, but now I'm getting this kind of error:

$ git archive-all --prefix=ppsspp-1.7.2/ ../SOURCES/ppsspp-1.7.2.tar.xz
[Errno 2] No such file or directory: '/home/akien/Mageia/Checkout/ppsspp/ppsspp/pspautotests/pspautotests'

I initially thought that it would be a git-archive-all regression and went back to older versions to test (down to 1.15), but all versions give me the same output.

I suspect this might be due to a change in Git itself (I'm running Git 2.19.1 now, but last time I used this workflow successfully I was on 2.13.x).

It used to work fine so IMO it could be handled as a bug in git-archive-all that it no longer works, but I'm fine with it being treated as a feature request to add a --ignore-unitialized switch or similar to enable this workflow. Thanks!

[Kentzo]

What if in addition to --extra there will be an --exclude option?

[akien-mga]

That would work for me too.

[Kentzo]

Added both options, so you should be able to exclude the submodules either way. Please try both of them and let me know if it works for your use-case.