a custom Git::Repository subclass to parse repository content

not sure if there's a real use for this class as a standalone module yet
add a 'kind' key to the hashes returned by the parse_... methods

parse_object now returns a single hash reference, which is a better interface
than the previous ( $kind => $hash ) one
add a commit with no message
do not drop trailing null fields
keep the mergetag headers in the original order
encoding is a single value header
typo fix
add a tag pointing to a tree (the empty tree)
add a tag pointing to a blob