merge: introduce 'c' action like 'g', but with additional safety _checkunknownfile() reads the filelog of the remote side's file. For narrow clones, the filelog will not exist for all files and we need a way to avoid reading them. While it would be easier for the narrow extension to just override _checkunknownfile() and make it ignore files outside the narrow clone, it seems cleaner to have manifestmerge() not care about filelogs (considering its name). In order to move the calls to _checkunknownfile() out, we need to be able to tell in which cases we should check for unknown files. Let's start by introducing a new action distinct from 'g' for this purpose. Specifically, the new action will be just like 'g' except that it will check that for conflicting unknown files first. For now, just add the new action type and convert it to 'g'.