dir: treat_leading_path() and read_directory_recursive(), round 2

I was going to title this "dir: more synchronizing of
treat_leading_path() and read_directory_recursive()", a nod to commit
777b42034764 ("dir: synchronize treat_leading_path() and
read_directory_recursive()", 2019-12-19), but the title was too long.

Anyway, first the backstory...

fill_directory() has always had a slightly error-prone interface: it
returns a subset of paths which *might* match the specified pathspec; it
was intended to prune away some paths which didn't match the specified
pathspec and keep at least all the ones that did match it.  Given this
interface, callers were responsible to post-process the results and
check whether each actually matched the pathspec.

builtin/clean.c did this.  It would first prune out duplicates (e.g. if
"dir" was returned as well as all files under "dir/", then it would
simplify this to just "dir"), and after pruning duplicates it would
compare the remaining paths to the specified pathspec(s).  This
post-processing itself could run into problems, though, as noted in
commit 404ebceda01c ("dir: also check directories for matching
pathspecs", 2019-09-17):

    For the case of git-clean and a set of pathspecs of "dir/file" and
    "more", this caused a problem because we'd end up with dir entries
    for both of
      "dir"
      "dir/file"
    Then correct_untracked_entries() would try to helpfully prune
    duplicates for us by removing "dir/file" since it's under "dir",
    leaving us with
      "dir"
    Since the original pathspec only had "dir/file", the only entry left
    doesn't match and leaves nothing to be removed.  (Note that if only
    one pathspec was specified, e.g. only "dir/file", then the
    common_prefix_len optimizations in fill_directory would cause us to
    bypass this problem, making it appear in simple tests that we could
    correctly remove manually specified pathspecs.)

That commit fixed the issue -- when multiple pathspecs were specified --
by making sure fill_directory() wouldn't return both "dir" and
"dir/file" outside the common_prefix_len optimization path.  This is
where it starts to get fun.

In commit b9670c1f5e6b ("dir: fix checks on common prefix directory",
2019-12-19), we noticed that the common_prefix_len wasn't doing
appropriate checks and letting all kinds of stuff through, resulting in
recursing into .git/ directories and other craziness.  So it started
locking down and doing checks on pathnames within that code path.  That
continued with commit 777b42034764 ("dir: synchronize
treat_leading_path() and read_directory_recursive()", 2019-12-19), which
noted the following:

    Our optimization to avoid calling into read_directory_recursive()
    when all pathspecs have a common leading directory mean that we need
    to match the logic that read_directory_recursive() would use if we
    had just called it from the root.  Since it does more than call
    treat_path() we need to copy that same logic.

...and then it more forcefully addressed the issue with this wonderfully
ironic statement:

    Needing to duplicate logic like this means it is guaranteed someone
    will eventually need to make further changes and forget to update
    both locations.  It is tempting to just nuke the leading_directory
    special casing to avoid such bugs and simplify the code, but
    unpack_trees' verify_clean_subdirectory() also calls
    read_directory() and does so with a non-empty leading path, so I'm
    hesitant to try to restructure further.  Add obnoxious warnings to
    treat_leading_path() and read_directory_recursive() to try to warn
    people of such problems.

You would think that with such a strongly worded description, that its
author would have actually ensured that the logic in
treat_leading_path() and read_directory_recursive() did actually match
and that *everything* that was needed had at least been copied over at
the time that this paragraph was written.  But you'd be wrong, I messed
it up by missing part of the logic.

Copy the missing bits to fix the new final test in t7300.

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

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