"git filter-branch" cannot reproduce a history with a tag without
the tagger field, which only ancient versions of Git allowed to be
created. This has been corrected.
* ic/fix-filter-branch-to-handle-tag-without-tagger:
filter-branch: use hash-object instead of mktag
filter-branch: stash away ref map in a branch
filter-branch: preserve and restore $GIT_AUTHOR_* and $GIT_COMMITTER_*
filter-branch: reset $GIT_* before cleaning up