copyfiles: add a way to relax the file system checking for hardlink

This is critical for transaction file, less for hardlink/copy clone as we are
about to do. Since `pure` build does not have a `getfstype` implementation this
would disable hardlink clone for all pure build. So we add a parameter to
control that extra check.

Differential Revision: https://phab.mercurial-scm.org/D10854

mercurial/util.py

@@ -1917,6 +1917,7 b' def copyfile('
     checkambig=False,
     nb_bytes=None,
     no_hardlink_cb=None,
+    check_fs_hardlink=True,
 ):
     """copy a file, preserving mode and optionally other stat info like
     atime/mtime
@@ -1935,7 +1936,7 b' def copyfile('
         if checkambig:
             oldstat = checkambig and filestat.frompath(dest)
         unlink(dest)
-    if hardlink:
+    if hardlink and check_fs_hardlink:
         # Hardlinks are problematic on CIFS (issue4546), do not allow hardlinks
         # unless we are confident that dest is on a whitelisted filesystem.
         try: