Optimize searches for directory structures by keeping a pointer to struct directory...

* src/common.h (struct name): New member `directory' replaces
dir_contents. Rearrange members.
(rebase_directory): Change signature.
(scan_directory): Change signature.
(name_fill_directory)
(directory_contents, safe_directory_contents): New prototypes.
(append_incremental_renames): Change signature.
(replace_prefix): New proto.
* src/compare.c (diff_dumpdir): Use directory_contents + scan_directory.
* src/create.c
* src/incremen.c (replace_prefix): Move to misc.c
(rebase_directory): Rewrite.
(scan_directory): Return pointer to struct directory.
(directory_contents, safe_directory_contents): New functions.
(get_directory_contents): Remove.
(name_fill_directory): New function.
(append_incremental_renames): Rewrite. This also fixes a memory leak.
* src/names.c (name_gather, addname): Reflect changes in struct name.
(add_hierarchy_to_namelist): Rewrite using name_fill_directory and
directory_contents.
(rebase_child_list): Update call to rebase_directory.
(collect_and_sort_names): Optimize

* src/misc.c (replace_prefix): New function.
* src/names.c (add_hierarchy_to_namelist): Use new get_directory_contents.

* tests/incr05.at: New test case.
* tests/incr06.at: New test case.
* tests/Makefile.am, test/testsuite.at: Add incr05.at and incr06.at.

* doc/Makefile.am (check-options): Improve rule.
* doc/tar.texi, NEWS: Update.