less(1) is used to display those diffs, or possibly whatever viewer is set in
$PAGER. Only the diffs are shown.
The filename is shown in the comments at the top, or in the menu right above the prompt.
To see the full file, switch to another console or window and open it in an editor. Or use
m to merge the two, but read up on
sdiff(1) first.
(Oh, but this was about single user mode, where there aren't any other consoles or windows. Sorry, that limits options. I don't run
mergemaster(8) in single user mode, I do this:
Building FreeBSD World And Kernel: The Short Form.)
Incidentally,
/etc/master.passwd should be merged very, very carefully.
mergemaster(8) has a config file which can be set to ignore certain files, and that's one file I put in there. Mistaken changes to
/etc/master.passwd can be ...unpleasant.