dir, clean: avoid disallowed behavior

dir.h documented quite clearly that DIR_SHOW_IGNORED and
DIR_SHOW_IGNORED_TOO are mutually exclusive, with a big comment to this
effect by the definition of both enum values.  However, a command like
   git clean -fx $DIR
would set both values for dir.flags.  I _think_ it happened to work
because:
  * As dir.h points out, DIR_KEEP_UNTRACKED_CONTENTS only takes effect
    if DIR_SHOW_IGNORED_TOO is set.
  * As coded, I believe DIR_SHOW_IGNORED would just happen to take
    precedence over DIR_SHOW_IGNORED_TOO in the code as currently
    constructed.
Which is a long way of saying "we just got lucky".

Fix clean.c to avoid setting these mutually exclusive values at the same
time, and add a check to dir.c that will throw a BUG() to prevent anyone
else from making this mistake.

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

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