t4063: add tests of direct blob diffs

The git-diff command can directly compare two blobs (or a
blob and a file), but we don't test this at all. Let's add
some basic tests that reveal a few problems.

There are basically four interesting inputs:

  1. sha1 against sha1 (where diff has no information beyond
     the contents)

  2. tree:path against tree:path (where it can get
     information via get_sha1_with_context)

  3. Same as (2), but using the ".." range syntax

  4. tree:path against a filename

And beyond generating a sane diff, we care about a few
little bits: which paths they show in the diff header, and
whether they correctly pick up a mode change.

They should all be able to show a mode except for (1),
though note that case (3) is currently broken.

For the headers, we would ideally show the path within the
tree if we have it, making:

  git diff a:path b:path

look the same as:

  git diff a b -- path

We can't always do that (e.g., in the direct sha1/sha1 diff,
we have no path to show), in which case we should fall back
to the name that resolved to the blob (which is nonsense
from the repository's perspective, but is the best we can
do).

Aside from the fallback case in (1), none of the cases get
this right. Cases (2) and (3) always show the full
tree:path, even though we should be able to know just the
"path" portion.

Case (4) picks up the filename path, but assigns it to
_both_ sides of the diff. So this works for:

  git diff tree:path path

but not for:

  git diff tree:other_path path

The appropriate tests are marked to expect failure.

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

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