subrepo: add regression test for issue3870

subrepo: clone of git sub-repository creates incorrect git branch (issue3870) Mercurial handles git subrepos by incorrect way. If the mercurial repo has a git sub-repo and somebody started a new branch in the subrepo and push it into git, the next one who will clone the whole repo will get incorrect branch name in the git subrepo.

dispatch: print 'abort:' when a pre-command hook fails (BC) This also changes the exit code from whatever the hook returned to 255. This brings it in line with all the other hooks that abort.