support .gitattributes normalization

[pmrowla]

Follow up to #211

dulwich currently lacks support for .gitattributes line ending/normalization settings, we should either contribute this upstream or detect when text normalization attributes are set and then force the use of pygit2/libgit2 for generating commits.

[jelmer]

FWIW Dulwich now (0.24+) has support for line ending handling and git attributes.