bundle2: don't check for whether we can do stream clones At the moment this isn't used and all stream clones use the legacy protocol. In an upcoming diff, canperformstreamclone will print out a message if a stream clone was requested but couldn't happen for some reason. Removing this call ensures the message isn't printed twice.

debugcommands: add debugpickmergetool to examine which merge tool is chosen Before this patch, there is no convenient way to know which merge tool is chosen for each managed files without actual merging.

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.