cmdutil: replace unix pipe handshake with file lock
The pipe handshake cannot be implemented with Windows detached process, which
disables handle inheritance.
Fix 2/3 for issue421
adding a
adding b
adding c
% should succeed
200 Script output follows
/al/
/b/
/c/
200 Script output follows
a
200 Script output follows
b
200 Script output follows
c
% should fail
404 Not Found
error: repository circle not found
404 Not Found
error: repository circle not found
404 Not Found
error: repository circle not found
% collections errors