Bug? git submodule update --reference doesn't use the referenced repository

Hi there,

I have been trying to use the --reference option to clone a big repository using a local copy, but I can't manage to make it work using sub-module update. I believe this is a bug, unless I missed something. I am on Windows, Git 2.12.0

So the problem is as follow:
- I have got a repository with multiple sub-modules, say
     main
         lib1
             sub-module1.git
         lib2
             sub-module2.git
- The original repositories are in GitHub, which makes it slow
- I have done a normal git clone of the entire repository (not bare) and 
put it on a file server, say \\fileserver\ref_repo\
(Note that the problem also happens with local copy)

So if I do a clone to get the repo and all the submodules with... git clone --reference-if-able \\fileserver\ref-repo --recursive git@github.com:company/main ...then it all works, all the sub-modules get cloned and the it's fast.

Now in my case I am working with Jenkins jobs and I need to first do a clone, and then get the sub-modules, but if I do... git clone --reference-if-able \\fileserver\ref-repo git@github.com:company/main (so non-recursive) cd main git submodule update --init --reference \\fileserver\ref-repo ... then this takes ages, as it would normally do without the use of --reference. I suspect it's not actually using it. The git clone documentation mentions that the reference is then passed to the sub-module clone commands, so I would expect "git clone --recursive" to work the same as "git submodule update", as far as --reference is concerned.

I noticed for a single module, doing a... git submodule update --init --reference \\fileserver\ref-repo\lib1\sub-module1 -- lib1/sub-module1 ...i.e. adding the sub-module path to the reference path, works. Which kind of make sense but then how do you do to apply it to all the sub-modules? (without writing a script to do that)

If someone can confirm the problem or explain me what I am dong wrong that would be great.

Maxime