util: add allowhardlinks module variable

To enable extensions to enable hardlinks for certain environments, let's move
the 'if False' to be an 'if allowhardlinks' and let extensions modify the
allowhardlinks variable.

Tests on linux ext4 pass with it set to True and to False.

mercurial/util.py

@@ -1056,6 +1056,11 b' def checksignature(func):'
 
     return check
 
+# Hardlinks are problematic on CIFS, do not allow hardlinks
+# until we find a way to work around it cleanly (issue4546).
+# This is a variable so extensions can opt-in to using them.
+allowhardlinks = False
+
 def copyfile(src, dest, hardlink=False, copystat=False, checkambig=False):
     '''copy a file, preserving mode and optionally other stat info like
     atime/mtime
@@ -1072,9 +1077,7 b' def copyfile(src, dest, hardlink=False, '
         if checkambig:
             oldstat = checkambig and filestat(dest)
         unlink(dest)
-    # hardlinks are problematic on CIFS, quietly ignore this flag
-    # until we find a way to work around it cleanly (issue4546)
-    if False and hardlink:
+    if allowhardlinks and hardlink:
         try:
             oslink(src, dest)
             return