Timestamp fixes for undo.
* doc/lispref/text.texi (Undo): Document (t . 0) and (t . -1) in buffer-undo-list. * etc/NEWS: Changes to visited-file-modtime, set-visited-file-modtime. * lisp/files.el (clear-visited-file-modtime): Move here from fileio.c. * src/atimer.c (schedule_atimer): * src/fileio.c (Ffile_newer_than_file_p): Minor cleanup: use EMACS_TIME_LT so that we can remove EMACS_TIME_GT. * src/buffer.c (buffer-undo-list): Document (t . 0) and (t . -1). * src/fileio.c (Fclear_visited_file_modtime): Move to lisp/files.el. (syms_of_fileio): Remove Sclear_visited_file_name. (Fvisited_file_modtime): Return -1, not (-1 ...), when the visited file doesn't exist; this avoids an ambiguity with negative timestamps. (Fset_visited_file_modtime): Accept -1 and 0 as time-list arg. * src/systime.h (make_emacs_time, invalid_emacs_time): Don't assume struct timespec layout; POSIX doesn't guarantee it. (EMACS_TIME_NE, EMACS_TIME_GT, EMACS_TIME_GE): Remove. * src/undo.c (record_first_change): Push (visited-file-modtime) onto undo list rather than reimplementing it by hand, incorrectly. Fixes: debbugs:14824
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