test-revset: add a few more tests of descendants() I'll add depth support to descendants().

dagop: unnest inner generator of revdescendants() This just moves iterate() to module-level function.

smartset: fix default value of abstractsmartset.sort() It's unused, but it shouldn't lie.

keyword: wrap functions only once at loading keyword extension Before this patch, some functions are wrapped in reposetup(), but this causes redundant nested wrapping, if two ore more repositories enable keyword extension (e.g. hgweb serves multiple repositories). Now, there is no need to define these wrapper functions in reposetup(), because previous patches made them not directly refer to kwtemplater instanciated in reposetup(). This patch factors these wrapper functions out from reposetup(), and uses them to wrap functions only at once at loading keyword extension.