diff: always try to set up the repository

diff: always try to set up the repository

If we see an explicit "--no-index", we do not bother calling
setup_git_directory_gently() at all. This means that we may
miss out on reading repo-specific config.

It's arguable whether this is correct or not. If we were
designing from scratch, making "git diff --no-index"
completely ignore the repository makes some sense. But we
are nowhere near scratch, so let's look at the existing
behavior:

  1. If you're in the top-level of a repository and run an
     explicit "diff --no-index", the config subsystem falls
     back to reading ".git/config", and we will respect repo
     config.

  2. If you're in a subdirectory of a repository, then we
     still try to read ".git/config", but it generally
     doesn't exist. So "diff --no-index" there does not
     respect repo config.

  3. If you have $GIT_DIR set in the environment, we read
     and respect $GIT_DIR/config,

  4. If you run "git diff /tmp/foo /tmp/bar" to get an
     implicit no-index, we _do_ run the repository setup,
     and set $GIT_DIR (or respect an existing $GIT_DIR
     variable). We find the repo config no matter where we
     started, and respect it.

So we already respect the repository config in a number of
common cases, and case (2) is the only one that does not.
And at least one of our tests, t4034, depends on case (1)
behaving as it does now (though it is just incidental, not
an explicit test for this behavior).

So let's bring case (2) in line with the others by always
running the repository setup, even with an explicit
"--no-index". We shouldn't need to change anything else, as the
implicit case already handles the prefix.

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