pager: further simplify code, clean up comments

Use the pager given by the environment to display long output Unix systems usually have a PAGER environment variable set. If it is set, mercurial will use the pager application to display output. Two configuration variables are available to influence the behaviour of the pager: pager.application sets the application to be used pager.quiet silences Broken Pipe errors that might occur when the user quits the pager before mercurial finished to write the output

hgk: don't exit if mercurial commands only print warnings