hghave: add a check about whitelisted filesystem that supports hardlink

This is needed for the test added by the next patch.

tests/hghave.py

@@ -346,6 +346,12 b' def has_hardlink():'
     finally:
         os.unlink(fn)
 
+@check("hardlink-whitelisted", "hardlinks on whitelisted filesystems")
+def has_hardlink_whitelisted():
+    from mercurial import osutil, util
+    fstype = getattr(osutil, 'getfstype', lambda x: None)('.')
+    return fstype in util._hardlinkfswhitelist
+
 @check("rmcwd", "can remove current working directory")
 def has_rmcwd():
     ocwd = os.getcwd()