util: enable hardlink for copyfile This patch removes the global variable "allowhardlinks" that disables hardlink in all cases, so hardlink gets enabled if the filesystem type is whitelisted. Third party extensions wanting to enable hardlink support unconditionally can replace "_hardlinkfswhitelist.__contains__".

hghave: add a check about whitelisted filesystem that supports hardlink This is needed for the test added by the next patch.

util: disable hardlink for copyfile if fstype is outside a whitelist Since osutil.getfstype is available, use it to detect filesystem types. The whitelist currently includes common local filesystems on Linux where they should have good hardlink support. We may add new filesystems for other platforms later.