SHA256 support

[lel-amri]

DRAFT

[lel-amri]

Attempt to fix #559. WIP (this comment will be rewritten). Missing tests. Have to clean some parsing done prior to object format detection.

I went for detection of object format from the git fast-export output. This is the hardest way. There is an easier one that should also yield cleaner code that would be to check repo's objectFormat config value prior to parsing, and thus set the parser to the expected object format at initialization. What do you think @newren?