filter-branch: use hash-object instead of mktag

filter-branch: use hash-object instead of mktag

This allows us to recreate even historical tags which would now be consider
invalid, such as v2.6.12-rc2..v2.6.13-rc3 in the Linux kernel source tree which
lack the `tagger` header.

    $ git rev-parse v2.6.12-rc2
    9e734775f7c22d2f89943ad6c745571f1930105f
    $ git cat-file tag v2.6.12-rc2 | git mktag
    error: char76: could not find "tagger "
    fatal: invalid tag signature file
    $ git cat-file tag v2.6.12-rc2 | git hash-object -t tag -w --stdin
    9e734775f7c22d2f89943ad6c745571f1930105f

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>