Replace invalid bytes when decoding TagObject stream

Fixes #943

Author: Harmon758

Diff for: doc/source/changes.rst

@@ -13,6 +13,8 @@ Bugfixes
 
 * Changed to use UTF-8 instead of default encoding when getting information about a symbolic reference
   (`#774 <https://github.com/gitpython-developers/GitPython/issues/774>`_)
+* Fixed decoding of tag object message so as to replace invalid bytes
+  (`#943 <https://github.com/gitpython-developers/GitPython/issues/943>`_)
 
 3.0.8
 =====

Diff for: git/objects/tag.py

@@ -50,7 +50,7 @@ def _set_cache_(self, attr):
         """Cache all our attributes at once"""
         if attr in TagObject.__slots__:
             ostream = self.repo.odb.stream(self.binsha)
-            lines = ostream.read().decode(defenc).splitlines()
+            lines = ostream.read().decode(defenc, 'replace').splitlines()
 
             _obj, hexsha = lines[0].split(" ")
             _type_token, type_name = lines[1].split(" ")