"warning: conflicting chdir during another chdir block" start showing up in 1.4.0+ version

[gyfelton]

This is esp. bad if there are multiple submodules for a repo. But it will show up at least once for every git fastclone operation
I believe the real reason is Dir.chdir is not thread safe: https://bugs.ruby-lang.org/issues/9785 but we perform this in order to checkout each module in separate thread. So far not causing any problem because we don't really do much git operation outside the chdir block after we start different threads.
Just to put out the problem here for discussion on how to resolve it.