cat-file tests: test messaging on bad objects/paths

cat-file tests: test messaging on bad objects/paths

Add tests for the output that's emitted when we disambiguate
<obj>:<path> in cat-file. This gives us a baseline for improving these
messages.

For e.g. "git blame" we'll emit:

    $ git blame HEAD:foo
    fatal: no such path 'HEAD:foo' in HEAD

But cat-file doesn't disambiguate these two cases, and just gives the
rather unhelpful:

    $ git cat-file --textconv HEAD:foo
    fatal: Not a valid object name HEAD:foo

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>