merge: collect checking for unknown files at end of manifestmerge() The 'c' and 'dc' actions include creating a file on disk and we need to check that no conflicting file exists unless force=True. Move two of the calls to _checkunknownfile() to a single place at the end of manifestmerge(). This removes some of the reading of filelogs from the heart of manifestmerge() and collects it in one place close to where its output (entries in the 'aborts' list) is used. Note that this removes the unnecessary call to _checkunknownfile() when force=True in one of the code paths.