git (post-receive-alternativearch): use unicode for git output
    
    The commit message and diff is stored in `full_change` as a string, via
    `read_output()`.  This is passed to `TEXT` which is a unicode string.
    
    When a commit message or diff contains non-ascii characters we get:
    
        UnicodeDecodeError: 'ascii' codec can't decode byte ...
    
    Encode git output returned from `read_output()` as a unicode string and
    define `full_change` as unicode for completeness.
    
    Fixes #6040.