Add test case as example of Git commit with invalid data

This is a real commit from the microjs.com open source project, see
microjs/microjs.com@7e8457c,
which is declared to be encoded in UTF-8, but contains invalid bytes.

This makes GitPython choke on it while decoding.  Rather than choking,
this should instead accept the error and replace the invalid bytes by
the �  (\x80) char.

Author: nvie

git/test/fixtures/commit_invalid_data

@@ -0,0 +1,6 @@
+tree 9f1a495d7d9692d24f5caedaa89f5c2c32d59368
+parent 492ace2ffce0e426ebeb55e364e987bcf024dd3b
+author E.Azer Ko�o�o�oculu <[email protected]> 1306710073 +0300
+committer E.Azer Ko�o�o�oculu <[email protected]> 1306710073 +0300
+
+add environjs