revert: properly back up added files with local modification These files were previously not backed up because the backup mechanism was not smart enough. This leads to data lose for the user since uncommitted contents were discarded. We now properly move the modified version to <filename>.orig before deleting it. We have to use a small hack to do a different action if "--no-backup" is specified. This is needed because the backup process is actually a move (not a copy) so the file is already missing when we backup. The internet kitten is a bit disapointed about that, but such is life. This patch concludes the "lets refactor revert" phases. We can now open the "Lets find stupid bug with renames and merge" phases. I'm sure that now that the code is clearer we could do it in another simpler way, but I consider the current improvement good enough for now.

revert: track added files with local modifications Those files need to be backed up but are currently not. We compute the set of them to be able to use a different backup strategy in the next changeset.

revert: distinguish between "check" and "backup" strategy "check" behaves as backup did before. We check if the current file differs from destination and we create a backup if it does. This is used for untracked files that will be overwritten by formerly-deleted files. We have to do the manual check since no status output can provide the content comparison. "backup" is now doing unconditional backup. This can be used for files seen as modified compared to both the target and the working directory. In such a case, we know that the file differs from target without actually comparing any content. This new "backup" strategy will be especially useful in the case of files added between the target and the working directory -parent- with additional modifications in the working directory -itself-. In that case we know we need to back it up, but we cannot run the content check as the files does not exists in target.