This change adds back support for line scrubbing and
adds a column that displays the time offset from
different points in the file. Also, try to handle
deleted files by dropping their lines from the display
view.
* help.txt: Update the help text for the new features.
* lnav.cc: Add scrub/time-offset key bindings. Also
added some more comments and added some handling for
deleted files.
* log_format_impls.cc: Add scrubbing for the generic
log format that shortens timestamps and RDNS strings.
* logfile_sub_source.cc: Add a time-offset column to
the display.
* textview_curses.cc: Some cleanup.
