Version 1.20.

* configure.ac: Raise version number to 1.20
* src/compare.c (diff_dumpdir): const.
* src/common.h (dumpdir_t,dumpdir_iter_t): New data types.
(dumpdir_create0,dumpdir_create,dumpdir_free,dumpdir_locate)
(dumpdir_first,dumpdir_next): New functions.
* src/incremen.c (dumpdir_create0,dumpdir_create,dumpdir_free)
(dumpdir_first,dumpdir_next): New functions.
(dumpdir_locate): Rewrite using binary search.
(struct directory): Change members char *contents, *icontents to
struct dumpdir *dump, *idump. All references updated.
(note_directory): Last arg is const.
* src/names.c (add_hierarchy_to_namelist): buffer is const.
* tests/incr03.at, tests/incr04.at, tests/rename02.at,
tests/rename03.at: Insert calls to sleep between creation of files
and adding them to the archive.