completion: mark bash script as LF-only

Without this change, the completion script does not work, as Bash expects
its scripts to have line feeds as end-of-line markers (this is
particularly prominent in quoted multi-line strings, where carriage
returns would slip into the strings as verbatim characters otherwise).

This change is required to let t9902-completion pass when Git's source
code is checked out with `core.autocrlf = true`.

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/completion/.gitattributes

@@ -0,0 +1 @@
+*.bash eol=lf