clean: avoid removing untracked files in a nested git repository

Users expect files in a nested git repository to be left alone unless
sufficiently forced (with two -f's).  Unfortunately, in certain
circumstances, git would delete both tracked (and possibly dirty) files
and untracked files within a nested repository.  To explain how this
happens, let's contrast a couple cases.  First, take the following
example setup (which assumes we are already within a git repo):

   git init nested
   cd nested
   >tracked
   git add tracked
   git commit -m init
   >untracked
   cd ..

In this setup, everything works as expected; running 'git clean -fd'
will result in fill_directory() returning the following paths:
   nested/
   nested/tracked
   nested/untracked
and then correct_untracked_entries() would notice this can be compressed
to
   nested/
and then since "nested/" is a directory, we would call
remove_dirs("nested/", ...), which would
check is_nonbare_repository_dir() and then decide to skip it.

However, if someone also creates an ignored file:
   >nested/ignored
then running 'git clean -fd' would result in fill_directory() returning
the same paths:
   nested/
   nested/tracked
   nested/untracked
but correct_untracked_entries() will notice that we had ignored entries
under nested/ and thus simplify this list to
   nested/tracked
   nested/untracked
Since these are not directories, we do not call remove_dirs() which was
the only place that had the is_nonbare_repository_dir() safety check --
resulting in us deleting both the untracked file and the tracked (and
possibly dirty) file.

One possible fix for this issue would be walking the parent directories
of each path and checking if they represent nonbare repositories, but
that would be wasteful.  Even if we added caching of some sort, it's
still a waste because we should have been able to check that "nested/"
represented a nonbare repository before even descending into it in the
first place.  Add a DIR_SKIP_NESTED_GIT flag to dir_struct.flags and use
it to prevent fill_directory() and friends from descending into nested
git repos.

With this change, we also modify two regression tests added in commit
91479b9c72f1 ("t7300: add tests to document behavior of clean and nested
git", 2015-06-15).  That commit, nor its series, nor the six previous
iterations of that series on the mailing list discussed why those tests
coded the expectation they did.  In fact, it appears their purpose was
simply to test _existing_ behavior to make sure that the performance
changes didn't change the behavior.  However, these two tests directly
contradicted the manpage's claims that two -f's were required to delete
files/directories under a nested git repository.  While one could argue
that the user gave an explicit path which matched files/directories that
were within a nested repository, there's a slippery slope that becomes
very difficult for users to understand once you go down that route (e.g.
what if they specified "git clean -f -d '*.c'"?)  It would also be hard
to explain what the exact behavior was; avoid such problems by making it
really simple.

Also, clean up some grammar errors describing this functionality in the
git-clean manpage.

Finally, there are still a couple bugs with -ffd not cleaning out enough
(e.g.  missing the nested .git) and with -ffdX possibly cleaning out the
wrong files (paying attention to outer .gitignore instead of inner).
This patch does not address these cases at all (and does not change the
behavior relative to those flags), it only fixes the handling when given
a single -f.  See
https://public-inbox.org/git/20190905212043.GC32087@szeder.dev/ for more
discussion of the -ffd[X?] bugs.

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

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