t9103: modernize test style

The main goal here was to avoid double-quotes for surrounding the test snippet, since it makes the code hard to read (and to grep for common problems). But while we're here, we can fix a few other things: - use test_path_* helpers, which are more robust and give better error messages - only "cd" inside a subshell, which leaves the environment pristine if further tests are added - consistently quote shell arguments. These aren't wrong if we assume find-rev output doesn't have any whitespace, but it doesn't hurt to be careful. - replace the old-style 'test x$foo = x' with 'test -z "$foo"'. Besides the quoting fix, this is the form we generally use in our test suite. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>