icasefs: rewrite case-folding collision detection (issue3452) Before this patch, case-folding collision detection uses "copies.pathcopies()" before "manifestmerge()", and is not aware of renaming in some cases. For example, in the case of issue3452, "copies.pathcopies()" can't detect renaming, if the file is renamed at the revision before common ancestor of merging. So, "hg merge" is aborted unexpectedly on case insensitive filesystem. This patch fully rewrites case-folding collision detection, and relocate it into "manifestmerge()". New implementation uses list of actions held in "actions" and "prompts" to build provisional merged manifest up. Provisional merged manifest should be correct, if actions required to build merge result up in working directory are listed up in "actions" and "prompts" correctly. This patch checks case-folding collision still before prompting for merge, to avoid aborting after some interactions with users. So, this assumes that user would choose not "deleted" but "changed". This patch also changes existing abort message, because sorting before collision detection changes order of checked files.