Update the comment about --mixed and paths

This updates the comment in HEAD.reset about why `--mixed` is
omitted from the git command constructed to perform a reset where
paths are being passed, adding specific information about the git
versions where this is deprecated, and changing the more-info link
from an old GitPython issue that is no longer retrievable to #1876.

Author: EliahKagan

git/refs/head.py

@@ -99,8 +99,8 @@ def reset(
         if index:
             mode = "--mixed"
 
-            # It appears some git versions declare mixed and paths deprecated.
-            # See http://github.com/Byron/GitPython/issues#issue/2.
+            # Explicit "--mixed" when passing paths is deprecated since git 1.5.4.
+            # See https://github.com/gitpython-developers/GitPython/discussions/1876.
             if paths:
                 mode = None
             # END special case