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

[Maxime Viargues <maxime.viargues@serato.com>]

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