filemerge: move decorator definition for internal merge tools to registrar This patch also adds extra loading entry for internal merge tools to extensions.py, for similarity to other decorators defined in registrar.py. This patch uses "internalmerge" for decorator class name, instead of original "internaltool", because the latter is too generic. BTW, after this patch, 4-spaces indentation is added to the 1st line of internal merge tool description docstring, and this may make already translated entries in *.po fuzzy. Even though this indentation is required for "definition list" in reST syntax, absence of it has been overlooked, because help.makeitemsdoc() forcibly inserts it at generation of online help. But this forcible insertion causes formatting issue (I'll send another patch series for this). Therefore, this additional indentation should be reasonable.