update-index: add --clear option

This just discards existing entries from the index, which
can be useful if you are rewriting all entries with
"--index-info" or similar.
docs/filter-branch: clean up newsubdir example

Over the years, this simple example has ended up quite hard
to read because of the number of special cases that must be
handled. Let's simplify it a bit:

  1. Use the new "index-info --clear" to avoid the need for
     a temporary index.

  2. Use "-z" and "perl -0" to avoid dealing with quoting
     issues. As a bonus, using perl means that "\t" will
     work consistently in regexps (the previous example
     using sed was reported to fail on OS X).

  3. Change the indentation to keep one logical unit per
     line and avoid extra backslash-escaping.