Merge branch 'jc/maint-filter-branch-epoch-date'
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. Because of this, "filter-branch" failed
to rewrite commits with ancient timestamps.
* 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