Take into account chdir_fd when extracting from incremental dumps.

Take into account chdir_fd when extracting from incremental dumps.

* src/common.h (tar_savedir): New prototype.
* src/misc.c (tar_savedir): New function.
(remove_any_file): Use tar_savedir.
* src/incremen.c (try_purge_directory): Use tar_savedir.
* src/update.c (update_archive): Use tar_savedir.
* tests/incr07.at: New testcase.
* tests/Makefile.am (TESTSUITE_AT): Add new test.
* tests/testsuite.at: Likewise.
* THANKS: Updated.