tmux: Scrollback timestamp

When in copy mode the scrollback timestamp is displayed in addition to the scrollback line number in a format like: {timestamp} [{lines_from_end}/{history_total}]

The timestamp format depends on when the last line of history displayed was captured:

• hh:mm:ss [X/Y] if on the same calendar day
• DddDD [X/Y] if on a different calendar day

For example, if the pane has 1042 lines of history captured and you scrollback 10 lines to display a line that was captured on Wed 23 13:14:16 then tmux will display something like:

• 13:14:16 [10/1042] if on the same calendar day
• Wed23 [10/1042] if on a different calendar day

The feature arrived in v3.4. Unfortunately the code for this feature (window_copy_write_line) doesn't offer any opportunity to configure that at the moment.