clean: disambiguate the definition of -d

The -d flag pre-dated git-clean's ability to have paths specified.  As
such, the default for git-clean was to only remove untracked files in
the current directory, and -d existed to allow it to recurse into
subdirectories.

The interaction of paths and the -d option appears to not have been
carefully considered, as evidenced by numerous bugs and a dearth of
tests covering such pairings in the testsuite.  The definition turns out
to be important, so let's look at some of the various ways one could
interpret the -d option:

  A) Without -d, only look in subdirectories which contain tracked
     files under them; with -d, also look in subdirectories which
     are untracked for files to clean.

  B) Without specified paths from the user for us to delete, we need to
     have some kind of default, so...without -d, only look in
     subdirectories which contain tracked files under them; with -d,
     also look in subdirectories which are untracked for files to clean.

The important distinction here is that choice B says that the presence
or absence of '-d' is irrelevant if paths are specified.  The logic
behind option B is that if a user explicitly asked us to clean a
specified pathspec, then we should clean anything that matches that
pathspec.  Some examples may clarify.  Should

   git clean -f untracked_dir/file

remove untracked_dir/file or not?  It seems crazy not to, but a strict
reading of option A says it shouldn't be removed.  How about

   git clean -f untracked_dir/file1 tracked_dir/file2

or

   git clean -f untracked_dir_1/file1 untracked_dir_2/file2

?  Should it remove either or both of these files?  Should it require
multiple runs to remove both the files listed?  (If this sounds like a
crazy question to even ask, see the commit message of "t7300: Add some
testcases showing failure to clean specified pathspecs" added earlier in
this patch series.)  What if -ffd were used instead of -f -- should that
allow these to be removed?  Should it take multiple invocations with
-ffd?  What if a glob (such as '*tracked*') were used instead of
spelling out the directory names?  What if the filenames involved globs,
such as

   git clean -f '*.o'

or

   git clean -f '*/*.o'

?

The current documentation actually suggests a definition that is
slightly different than choice A, and the implementation prior to this
series provided something radically different than either choices A or
B. (The implementation, though, was clearly just buggy).  There may be
other choices as well.  However, for almost any given choice of
definition for -d that I can think of, some of the examples above will
appear buggy to the user.  The only case that doesn't have negative
surprises is choice B: treat a user-specified path as a request to clean
all untracked files which match that path specification, including
recursing into any untracked directories.

Change the documentation and basic implementation to use this
definition.

There were two regression tests that indirectly depended on the current
implementation, but neither was about subdirectory handling.  These two
tests were introduced in commit 5b7570cfb41c ("git-clean: add tests for
relative path", 2008-03-07) which was solely created to add coverage for
the changes in commit fb328947c8e ("git-clean: correct printing relative
path", 2008-03-07).  Both tests specified a directory that happened to
have an untracked subdirectory, but both were only checking that the
resulting printout of a file that was removed was shown with a relative
path.  Update these tests appropriately.

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

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