Add reference for Git magic hash for the empty tree

Signed-off-by: ziad hany <ziadhany2016@gmail.com>

Author: ziadhany

vulnerabilities/importers/osv.py

@@ -422,6 +422,7 @@ def get_fixed_versions_and_commits(
                 continue
 
             # Git uses this magic hash for the empty tree
+            # See: http://colinschimmelfing.com/blog/gits-empty-tree/
             if introduced == "0":
                 introduced = "4b825dc642cb6eb9a060e54bf8d69288fbee4904"