filter-branch: Big syntax change; support rewriting multiple refs

We used to take the first non-option argument as the name for the new
branch.  This syntax is not extensible to support rewriting more than just
HEAD.

Instead, we now have the following syntax:

git filter-branch [<filter options>...] [<rev-list options>]

All positive refs given in <rev-list options> are rewritten.  Yes,
in-place.  If a ref was changed, the original head is stored in
refs/original/$ref now, for your inspecting pleasure, in addition to the
reflogs (since it is easier to inspect "git show-ref | grep original" than
to inspect all the reflogs).

This commit also adds the --force option to remove .git-rewrite/ and all
refs from refs/original/ before filtering.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>