if some invalid char in comment.
it should throw a UnicodeDecodeError in "stdout.decode(ENCODING)".
I think should ignore those invalid char .
git use 'UTF-8' as default encoding

seeing : Git/doc/git/html/git-log.html

--encoding=<encoding>
The commit objects record the encoding used for the log message in their encoding header; this option can be used to tell the command to re-code the commit log message in the encoding preferred by the user. For non plumbing commands this defaults to UTF-8.
when decode error,print the data and reason to stderr.
Merge pull request #52 from cyqw/master

add errors as 'ignore' to decode