t9107: use "return 1" instead of "exit 1"

t9107: use "return 1" instead of "exit 1"

When a test runs a loop, it cannot rely on the usual
&&-chaining to propagate a failure inside the loop; it needs
to break out with a failure signal. However, unless you are
in a subshell, doing so with "exit 1" will exit the entire
test script, not just the test snippet we are in (and cause
the harness to complain that test_done was never reached).

So the fundamental point of this patch is s/exit/return/.
But while we're there, let's fix a number of style and
readability issues:

  - snippets in double-quotes need an extra layer of quoting
    for their meta-characters; let's avoid that by using
    single quotes

  - accumulating loop output by appending to a file in each
    iteration is brittle, as it can be affected by content
    left in the file by earlier tests. Instead, it's better
    to redirect stdout for the whole loop, so we know the
    output only comes from that loop.

  - using "test -z" to check that diff output is empty is
    overly verbose; we can just ask diff to use --exit-code.

  - we can factor out long lists of refs to make it more
    obvious we're using the same ones in each loop

  - subshells are unnecessary when ending an &&-chain with
    "|| return 1"

  - minor style fixups like space-after-redirection, and
    "do" and "done" on their own lines

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>