new safer autocrlf handling for text=auto

Commit fd6cce9e introdoced the new safer autocrlf handling:
When core.autocrlf is true, files which have mixed line endings
in the repo are not changed when they are checked out.

Do the same for files that have the text=auto attribute:
When user A uses core.autocrlf=true, text files added on this machine
will be normalized in the repository.
When user B clones the repository, uses an editor which saves changed lines
with CRLF and unchanged lines with LF and does not use core.autocrlf,
this causes trouble for user A once the file is pushed from B to A:
files which are unnormalized are reported as changed.
Now user A can add an attribute file with text=auto to the repository and
re-normalize it.
Once the .gitattrinute file reaches user B via push/pull
(or users C via git clone), text files will stay normalized.