This implements a feature request from here¹ about having a function to
resolve all conflicts simultaneously. Although question author didn't
reply, but either way I think it's a useful functional. I needed it so
many times, but before stumbling upon this question I just didn't know
there are functions `smerge-keep-upper/base/lower`, and so ofc I never
though of writing a new one that would apply them to all conflicts.

It might be useful to make a function to do the same within a git-repo,
but for now let's have at least a function that does that within a
file.

1:
https://emacs.stackexchange.com/questions/80361/when-merging-conflicts-in-smerge-mode-how-to-select-mine-for-all-conflicts