merge: remove "case" comments The comments introduced in f05c182430a0 seems important ... but the context has apparently been lost and they do not have any value now.

merge: merge file flags together with file content The 'x' flag and the 'l' flag are very different. It is usually not a problem to change the 'x' flag of a normal file independent of the content, but one does not simply change the type of a file to 'l' independent of the content. This removes the fmerge function that merged both 'x' and 'l' independent of content early in the merge process. This correctly introduces some conflicts instead of silent incorrect merges. 3-way flag merge will now be done in the resolve process, right next to file content merge. Conflicts can thus be resolved with (slightly inconvenient) resolve commands like 'resolve f --tool internal:other'. It thus brings us closer to be able to re-solve manifest merge after the merge and avoid prompts during merge. This also removes the "conflicting flags for a - (n)one, e(x)ec or sym(l)ink?" prompt that nobody could answer and that made it easy to mix symlink targets and file contents up. Instead it will give a file merge where a sufficiently clever merge tool can help resolving the issue.

tests: better test coverage of merges of flags This makes existing problems with merges of symlinks and files more obvious and add test coverage for tricky merges without real common ancestors.