t9103: modernize test style

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>