Can you give us a test case that fails with the current code and works with your suggested patch?

On git level the testcase is provided in the git mailing list thread I have linked in my first post. Sorry, I should have stated that clearer.

On go level you can reproduce the failure as follows:

export GO15VENDOREXPERIMENT=1
go get -d github.com/toxeus/test_for_issue

or any other git repo that has submodules.

The error message looks then like this:

# cd .; git --git-dir=/home/filip/go/src/github.com/toxeus/test_for_issue/.git submodule update --init --recursive
No submodule mapping found in .gitmodules for path 'vendor/golang.org/x/crypto'
package github.com/toxeus/test_for_issue: exit status 1

I think this is a dupe of #12573

CL https://golang.org/cl/17974 mentions this issue.