osutil: export a "getfstype" method
This patch exports the "getfstype" method. So we can use it to enable
hardlinks for known safe filesystems.
The patch was tested manually via debugshell on a Linux system.
"mercurial.osutil.getfstype" works as expected. It's hard to mount
filesystem on user-space easily. I will add a test for real hardlink support
to indirectly test this patch, after turning on real hardlinks support for
certain whitelisted filesystems.