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Old submodules broken in 2.19rc1 and 2.19rc2

Submodules checked out with older versions of git not longer works in the 
latest 2.19 releases. A "git submodule update --recursive" command wil fail 
for each submodule with a line saying "fatal: could not open 
'<submodule>/.git' for writing> Is a directory.

I checked the release notes so far, and they do not say the old submodules 
from git 2.16- were no longer supported, so I assume it is a bug?