scmutil: abort if an empty revision is given to revpair() When using 'extdiff --patch' to check the changes in a rebase, 'precursors(x)' evaluated to an empty set because I forgot the --hidden flag, so the other revision was used as the replacement for the empty set. The result was the patch for the other revision was diffed against itself, and the tool saying there were no differences. That's misleading since the expected diff args were silently changed, so it's better to bail out. The other uses of scmutil.revpair() are commands.diff and commands.status, and it doesn't make sense to allow an empty revision there either. The code here was suggested by Yuya Nishihara.