git-new-workdir: mark script as LF-only

Bash does not handle scripts with CR/LF line endings correctly, therefore
they *have* to be forced to LF-only line endings.

Funnily enough, this fixes t3000-ls-files-others and
t1021-rerere-in-workdir when git.git was checked out with
core.autocrlf=true, as these test still use git-new-workdir (once `git
worktree` is no longer marked as experimental, both scripts probably
want to be ported to using that command instead).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/workdir/.gitattributes

@@ -0,0 +1 @@
+/git-new-workdir eol=lf