gitattributes: mark AsciiDoc files as LF-only

gitattributes: mark AsciiDoc files as LF-only

In a future commit, we'll move the AsciiDoc documentation files to the
".adoc" extension rather than the extension ".txt".  We need these files
to use only LF because they are read by generate-cmdlist.sh using the
read builtin.

If we allow CRLF here, the CR at the end of the line is treated as part
of the synopsis, since a POSIX shell doesn't consider it special like
LF.  In that case, we generate synopsis strings in C that contain a CR,
which the compiler does not like because it believes that the double
quote string terminator is missing, and as a consequence, compilation
fails.

Because we rely on LF-only endings here to compile successfully and we
want Git to continue to be able to compile on Windows, mark these files
as LF-only in the .gitattributes file.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>