merge: abort on file/directory case folding collisions (issue4892) File/directory case folding collisions cannot be represented on case folding systems and have to fail. To detect this and abort early, utilize that for file/directory collisions, a sorted list of case folded manifest names will have the colliding directory right after the file. (This could perhaps be optimized, but this way of doing it also has directory/directory case folding in mind ... which however not is handled yet.)

tests: test of case-folding merge collisions

copies: group bothnew with other sets

copies: rename renamedelete to renamedeleteset for clarity

copies: move _makegetfctx calls into checkcopies

copies: factor out setupctx into _makegetfctx This reduces the scope of mergecopies a bit

copies: avoid reference to c1/c2 in makectx