evolve: use registrar.templatekeyword to register template keyword functions
Now, using registrar.templatekeyword of Mercurial directly in evolve
extension should be safe enough. because it has been available since
Mercurial 3.8, and minimum Mercurial version for evolve extension is
3.8, too.
BTW, this also fixes an issue of "hg help templates" output that
template keywords "precursors" and "successors" are not displayed as
same as others, because they do not have ":KEYWORD:" prefix in their
docstring.