convert: unify the "auto" handling of CRLF

convert: unify the "auto" handling of CRLF

Before this change,
$ echo "* text=auto" >.gitattributes
$ echo "* eol=crlf" >>.gitattributes

would have the same effect as
$ echo "* text" >.gitattributes
$ git config core.eol crlf

Since the 'eol' attribute had higher priority than 'text=auto', this may
corrupt binary files and is not what most users expect to happen.

Make the 'eol' attribute to obey 'text=auto' and now
$ echo "* text=auto" >.gitattributes
$ echo "* eol=crlf" >>.gitattributes
behaves the same as
$ echo "* text=auto" >.gitattributes
$ git config core.eol crlf

In other words,
$ echo "* text=auto eol=crlf" >.gitattributes
has the same effect as
$ git config core.autocrlf true

and
$ echo "* text=auto eol=lf" >.gitattributes
has the same effect as
$ git config core.autocrlf input

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>