git-worktree(1), and Git shared repository can use a plain text file
named `.git` at the root of working tree, containing `gitdir: <path>` to
point to the real directory that has repository.
See: gitrepository-layout(5).
But, that directory is usually inaccessible inside chroot.
In order to support git-worktree(1) and Git shared repository, compute
SOURCE_DATE_EPOCH from outside of chroot, and the chroot will carry it
over to inside chroot.
I've only tested again xbps-uunshare(1).
2cd2b99e41