t7012: add a testcase demonstrating stash apply bugs in sparse checkouts

Applying stashes in sparse-checkouts, particularly when the patterns
used to define the sparseness have changed between when the stash was
created and when it is applied, has a number of bugs.  The primary
problem is that stashes are sometimes only partially applied.  In most
such cases, it does so silently without any warning or error being
displayed and with 0 exit status.

There are, however, a few cases when non-translated error messages are
shown and the stash application aborts early.  The first is when there
are files present despite the SKIP_WORKTREE bit being set, in which case
the error message shown is:

    error: Entry 'PATHNAME' not uptodate. Cannot merge.

The other situation is when a stash contains new files to add to the
working tree; in this case, the code aborts early but still has the
stash partially applied, and shows the following error message:

    error: NEWFILE: does not exist and --remove not passed
    fatal: Unable to process path NEWFILE

Add a test that can trigger all three of these problems.  Have it
carefully check that the working copy and SKIP_WORKTREE bits are as
expected after the stash application.  The test is currently marked as
expected to fail, but subsequent commits will implement the fixes and
toggle the expectation.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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