Opening the destination at the source's exact mode breaks when the
source lacks owner write (e.g. r-xr-xr-x): the create still works but
the overwrite path can't reopen such a destination for writing. Force
owner read+write while copying and let the trailing Chmod() put the
source mode back, which only ever adds owner bits so the group/other
bits stay as restrictive as the source throughout. Add a regression
test covering the restricted-mode and read-only-source cases.
dxbjavid
01678166a8