opener: force copy on 'a'ppend if nlinks() returns 0 (issue1922)

If pywin32 is not installed, 'os.lstat(pathname).st_nlink' is used for
nlinks(), which is always zero for all files on Windows.

To make sure we break up hardlinks if pywin32 is missing, we force
nlink = 2 if nlinks() returns < 1.

(this completely fixes issue1922)

mercurial/util.py

@@ -913,6 +913,8 b' class opener(object):'
                     # shares if the file is open.
                     fd = open(f)
                     nlink = nlinks(f)
+                    if nlink < 1:
+                        nlink = 2 # force mktempcopy (issue1922)
                     fd.close()
             except (OSError, IOError):
                 nlink = 0