filemerge: add the function to examine a capability of a internal tool For "symlink" and "binary" capabilities, _toolbool() can not examine these of internal merge tools strictly, because it examines only configurations in "merge-tools" section. Users can configure them explicitly as below for example, but this is not ordinary usage and not convenient: [merge-tools] :other.symlink = true :other.binary = true This patch adds hascapability() internal function, which can examine actual capabilities of a internal merge tool strictly. At this patch, hascapability() is still used with "strict=False". Subsequent patches use it with "strict=True".