Merge branch 'ic/fix-filter-branch-to-handle-tag-without-tagger'

"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