util.copyfiles: don't try os_link() again if it failed before

If the os_link() call on the first file in the directory fails [1],
we switch mode to using shutil.copy() for all remaining files.

[1] happens for example on Windows for every file when cloning from a UNC
path without specifying --pull.

mercurial/util.py

@@ -451,7 +451,7 b' def copyfiles(src, dst, hardlink=None):'
         for name, kind in osutil.listdir(src):
             srcname = os.path.join(src, name)
             dstname = os.path.join(dst, name)
-            copyfiles(srcname, dstname, hardlink)
+            hardlink = copyfiles(srcname, dstname, hardlink)
     else:
         if hardlink:
             try:
@@ -462,6 +462,8 b' def copyfiles(src, dst, hardlink=None):'
         else:
             shutil.copy(src, dst)
 
+    return hardlink
+
 class path_auditor(object):
     '''ensure that a filesystem path contains no banned components.
     the following properties of a path are checked: