sha1dc: ignore indent-with-non-tab whitespace violations

The upstream sha1dc code indents some lines with spaces.
While this doesn't match Git's coding guidelines, it's better
to leave this imported code untouched than to try to make it
match our style. However, we can use .gitattributes to tell
"diff --check" and "git am" not to bother us about it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>