checkout tests: index should be clean after dwim checkout

checkout tests: index should be clean after dwim checkout

Assert that whenever there's a DWIM checkout that the index should be
clean afterwards, in addition to the correct branch being checked-out.

The way the DWIM checkout code in checkout.[ch] works is by looping
over all remotes, and for each remote trying to find if a given
reference name only exists on that remote, or if it exists anywhere
else.

This is done by starting out with a `unique = 1` tracking variable in
a struct shared by the entire loop, which will get set to `0` if the
data reference is not unique.

Thus if we find a match we know the dst_oid member of
tracking_name_data must be correct, since it's associated with the
only reference on the only remote that could have matched our query.

But if there was ever a mismatch there for some reason we might end up
with the correct branch checked out, but at the wrong oid, which would
show whatever the difference between the two staged in the
index (checkout branch A, stage changes from the state of branch B).

So let's amend the tests (mostly added in) 399e4a1c56 ("t2024: Add
tests verifying current DWIM behavior of 'git checkout <branch>'",
2013-04-21) to always assert that "status" is clean after we run
"checkout", that's being done with "-uno" because there's going to be
some untracked files related to the test itself which we don't care
about.

In all these tests (DWIM or otherwise) we start with a clean index, so
these tests are asserting that that's still the case after the
"checkout", failed or otherwise.

Then if we ever run into this sort of regression, either in the
existing code or with a new feature, we'll know.

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