t7300: add testcases showing failure to clean specified pathspecs

t7300: add testcases showing failure to clean specified pathspecs

Someone brought me a testcase where multiple git-clean invocations were
required to clean out unwanted files:
  mkdir d{1,2}
  touch d{1,2}/ut
  touch d1/t && git add d1/t
With this setup, the user would need to run
  git clean -ffd */ut
twice to delete both ut files.

A little testing showed some interesting variants:
  * If only one of those two ut files existed (either one), then only one
    clean command would be necessary.
  * If both directories had tracked files, then only one git clean would
    be necessary to clean both files.
  * If both directories had no tracked files then the clean command above
    would never clean either of the untracked files despite the pathspec
    explicitly calling both of them out.

A bisect showed that the failure to clean out the files started with
commit cf424f5fd89b ("clean: respect pathspecs with "-d", 2014-03-10).
However, that pointed to a separate issue: while the "-d" flag was used
by the original user who showed me this problem, that flag should have
been irrelevant to this problem.  Testing again without the "-d" flag
showed that the same buggy behavior exists without using that flag, and
has in fact existed since before cf424f5fd89b.

Although these problems at first are perceived to be different (e.g.
never clearing out the requested files vs. taking multiple invocations
to get everything cleared out), they are actually just different
manifestations of the same problem.  The case with multiple directories
that have no tracked files is the more general case; solving it will
solve all the others.  So, I concentrate on it.  Add testcases showing
that multiple untracked files within entirely untracked directories
cannot be cleaned when specifying these files to git clean via
pathspecs.

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