copyfile: allow optional hardlinking

Some code paths use 'copyfiles' (full tree) for a single file to take advantage
of the best-effort-hard-linking parameter. We add similar parameter and logic
to 'copyfile' (single file) for this purpose.

The single file version have the advantage to overwrite the destination file if
it exists.

mercurial/util.py

@@ -716,10 +716,16 b' def checksignature(func):'
 
     return check
 
-def copyfile(src, dest):
+def copyfile(src, dest, hardlink=False):
     "copy a file, preserving mode and atime/mtime"
     if os.path.lexists(dest):
         unlink(dest)
+    if hardlink:
+        try:
+            oslink(src, dest)
+            return
+        except (IOError, OSError):
+            pass # fall back to normal copy
     if os.path.islink(src):
         os.symlink(os.readlink(src), dest)
     else: