tar: fix bug with -C and delayed setting of metadata

tar: fix bug with -C and delayed setting of metadata

* src/common.h (chdir_current): New decl.
* src/extract.c (struct delayed_set_stat, struct delayed_link):
New member change_dir.
(delay_set_stat, create_placeholder_file): Set it.
(apply_nonancestor_delayed_set_stat, apply_delayed_links): Use it.
(extract_link): Check that the links are all relative to the same
directory.
(extract_archive): Restore the current directory after
apply_nonancestor_delayed_set_stat has possibly changed it.
* src/misc.c (chdir_current): New external var; this used to
be the private static variable 'previous' inside chdir_dir.
All uses changed.
* tests/Makefile.am (TESTSUITE_AT): New test extrac10.at.
* tests/extrac10.at: New file.
* tests/testsuite.at: Include it.