t5300: modernize basic tests

t5300: modernize basic tests

The first set of tests in t5300 goes back to 2005, and doesn't use some
of our customary style and tools these days. In preparation for touching
them, let's modernize a few things:

  - titles go on the line with test_expect_success, with a hanging
    open-quote to start the test body

  - test bodies should be indented with tabs

  - opening braces for shell blocks in &&-chains go on their own line

  - no space between redirect operators and files (">foo", not "> foo")

  - avoid doing work outside of test blocks; in this case, we can stick
    the setup of ".git2" into the appropriate blocks

  - avoid modifying and then cleaning up the environment or current
    directory by using subshells and "git -C"

  - this test does a curious thing when testing the unpacking: it sets
    GIT_OBJECT_DIRECTORY, and then does a "git init" in the _original_
    directory, creating a weird mixed situation. Instead, it's much
    simpler to just "git init --bare" a new repository to unpack into,
    and check the results there. I renamed this "git2" instead of
    ".git2" to make it more clear it's a separate repo.

  - we can observe that the bodies of the no-delta, ref_delta, and
    ofs_delta cases are all virtually identical except for the pack
    creation, and factor out shared helper functions. I collapsed "do
    the unpack" and "check the results of the unpack" into a single
    test, since that makes the expected lifetime of the "git2" temporary
    directory more clear (that also lets us use test_when_finished to
    clean it up). This does make the "-v" output slightly less useful,
    but the improvement in reading the actual test code makes it worth
    it.

  - I dropped the "pwd" calls from some tests. These don't do anything
    functional, and I suspect may have been an aid for debugging when
    the script was more cavalier about leaving the working directory
    changed between tests.

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