If you want to be absolutely safe with git_message_prettify, you
can now pass a NULL pointer for the buffer and get back the number
of bytes that would be copied into the buffer.
This means that an error is a non-negative return code and a
success will be greater than zero from this function.
git_commit() and git_tag() no longer prettify the
message by default. This has to be taken care of
by the caller.
This has the nice side effect of putting the
caller in position to actually choose to strip
the comments or not.
