t7063: more thorough status checking

t7063: more thorough status checking

It turns out the t7063 has some testcases that even without using the
untracked cache cover situations that nothing else in the testsuite
handles.  Checking the results of
  git status --porcelain
both with and without the untracked cache, and comparing both against
our expected results helped uncover a critical bug in some dir.c
restructuring.

Unfortunately, it's not easy to run status and tell it to ignore the
untracked cache; the only knob we have is core.untrackedCache=false,
which is used to instruct git to *delete* the untracked cache (which
might also ignore the untracked cache when it operates, but that isn't
specified in the docs).

Create a simple helper that will create a clone of the index that is
missing the untracked cache bits, and use it to compare that the results
with the untracked cache match the results we get without the untracked
cache.

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