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.
