filemerge: add internal merge tool to dump files forcibly Internal merge tool :dump implies premerge. Therefore, files aren't dumped, if premerge runs successfully. This undocumented behavior might confuse users, if they want to always dump files. But just making :dump omit premerge might cause backward compatibility issue for existing automation. This patch adds new internal merge tool :forcedump, which works as same as :dump, but omits premerge always. Internal tools annotated with "nomerge" should merge "change and delete" correctly, but _forcedump() can't. Therefore, it is annotated with "mergeonly" to always omit premerge, even though it doesn't merge files actually. This patch also adds explanation about premerge to :dump, to clarify how :dump actually works. BTW, this patch specifies internal tools with "internal:" prefix in newly added test scenario in test-merge-tools.t, even though this prefix is already deprecated. This is only for similarity to other tests in test-merge-tools.t.