Fix extraction from concatenated incremental archives.

* src/common.h (remove_delayed_set_stat): New proto.
* src/extract.c (free_delayed_set_stat)
(remove_delayed_set_stat): New function.
(apply_nonancestor_delayed_set_stat): Use free_delayed_set_stat.
* src/misc.c (safer_rmdir): Remove delayed_set_stat entry
corresponding to the removed directory.
* tests/incr10.at: New test case.
* tests/Makefile.am: Add new test.
* tests/testsuite.at: Likewise.