Merge branch 'jc/maint-filter-branch-epoch-date' into maint

In 1.7.9 era, we taught "git rebase" about the raw timestamp format but we did not teach the same trick to "filter-branch", which rolled a similar logic on its own. * jc/maint-filter-branch-epoch-date: t7003: add test to filter a branch with a commit at epoch date.c: Fix off by one error in object-header date parsing filter-branch: do not forget the '@' prefix to force git-timestamp