status: add a failing test showing a core.untrackedCache bug

status: add a failing test showing a core.untrackedCache bug

The untracked cache gets confused when a directory is swapped out for
a file. It is easiest to reproduce this by swapping out a directory
with a symlink to another directory, and as the tests show the symlink
case is the only case we've found where "git status" will subsequently
report incorrect information, even though it's possible to otherwise
get the untracked cache into a state where its internal data
structures don't reflect reality.

In the symlink case, whatever files are inside the target of the
symlink will be incorrectly shown as untracked. This issue does not
happen if the symlink links to another file, only if it links to
another directory.

A stand-alone testcase for copying into a terminal:

    (
        rm -rf /tmp/testrepo &&
        git init /tmp/testrepo &&
        cd /tmp/testrepo &&
        mkdir x y &&
        touch x/a y/b &&
        git add x/a y/b &&
        git commit -msnap &&
        git rm -rf y &&
        ln -s x y &&
        git add y &&
        git commit -msnap2 &&
        git checkout HEAD~ &&
        git status &&
        git checkout master &&
        sleep 1 &&
        git status &&
        git status
    )

This will incorrectly show y/a as an untracked file. Both the "git
status" call right before "git checkout master" and the "sleep 1"
after the "checkout master" are needed to reproduce this, presumably
due to the untracked cache tracking on the basis of cached whole
seconds from stat(2).

When git gets into this state, a workaround to fix it is to issue a
one-off:

    git -c core.untrackedCache=false status

For the non-symlink case, the bug is that the output of
test-dump-untracked-cache should not include:

   /one/ 0000000000000000000000000000000000000000 recurse valid

It being in the output implies that cached traversal of root includes
the directory "one" which does not exist on disk anymore.

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