Add .gitattributes to tell git to use LF newlines

These hook scripts execute with bash and need LF newlines.  Set eol=lf
to get LF newlines even when core.autocrlf is enabled.  While at it,
also enable whitespace=indent-with-non-tab.

.gitattributes

+.git*            export-ignore
+*                eol=lf         whitespace=indent-with-non-tab