Commit cce8d6fdb introduces file t/t5100/nul, git tree is now incompatible with Cygwin (and probably Windows)

[Mark Levedahl <mlevedahl@gmail.com>]

Beginning with the referenced commit, the git project cannot be checked 
out on Cygwin (and I assume cannot be checked out on Windows using 
msysgit, though I have not verified this) as this commit introduces the 
file "t/5100/nul." On Windows, the file name "nul" is reserved, 
regardless of path, and cannot be created or deleted. It serves 
essentially the same function as /dev/null.

As a for instance of the troubles:

git>git checkout -f origin/master
Previous HEAD position was a2f5be5... Merge branch 
'jk/maint-send-email-compose' into maint
error: git-checkout-index: unable to create file t/t5100/nul (File exists)

As this commit is part of the published master branch, I am not sure the 
correct resolution: leaving this commit in place means that any commit 
between it and a commit fixing this will always cause an error on Cygwin 
/ Windows. Of course, it *is* on the published master branch.

Mark