t/lib-credential.sh: drop shebang line
The purpose of lib-credential.sh is to be sourced into other test
scripts. It doesn't need a "#!/bin/sh" line, as running it directly
makes no sense. Nor does it serve any real filetype documentation
purpose, as the file is clearly named with a ".sh" extension.
In the spirit of
c74c72034f (test: replace shebangs with descriptions in
shell libraries, 2013-11-25), let's replace it with a human-readable
description.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>