in tar dump, convert uid, gid of value -1 to zero

changelog/unreleased/issue-4103

@@ -0,0 +1,12 @@
+Bugfix: fix restic dump of tar file with 32-bit binary
+
+In restic up to 0.14.0, the restic dump from a 32-bit binary of a
+snapshot of standard input that was created in Windows has as a
+result a tar file whose content has a negative uid and gid. As a
+result, gnu tar exits with failure status whenever it tries to
+access such a tar file. With this fix, the tar file that is now
+dumped from a 32-bit binary has content with non-negative uid and
+gid.
+
+https://github.com/restic/restic/issues/4103
+https://github.com/restic/restic/pull/4104