support for non-sha1 file validators

[Jelmer Vernooij]

git, svn and mercurial don't use sha1 hashes of the file full text.

svn uses md5 sums and git uses sha1 checksums with a custom header.

it would be useful (and very beneficial to performance of foreign plugins) if bzr could support these other file validators.

Perhaps Tree.get_file_validator() which returns a tuple with a validator type and the contents?