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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.

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