Summary:
Our `rename` is not atomic:
```
def rename(src, dst):
"""Rename file src to dst, replacing dst if it exists"""
try:
os.rename(src, dst)
except OSError as e:
if e.errno != errno.EEXIST:
raise
unlink(dst)
# What if the process is interrupted here?
os.rename(src, dst)
```
However, the `MoveFileEx` Windows API provides a way to to
replace the existing file, thus eliminating the need to do `unlink`.
Unfortunately, it only works for files, not for dirs, therefore
we're introducing a new file-specific rename function.
Differential Revision: D12940555
fbshipit-source-id: a6749a9b16a285788de0f5c06d51a15c919166ce
