mercurial: add debugextensions command (issue4676) Add debugextensions command to help users debug their extension problems. If there are no extensions command prints nothing, otherwise it prints names of extension modules. If quiet or verbose option is not specified it prints(after extensions name) last version of mercurial in which given module was tested for non internal modules or not tested with user mercurial version. If verbose is specified it prints following information for every extension: extension name, import source, testedwith and buglink information. Extensions are printed sorted by extension name.