pushrace: avoid crash on bare push when using concurrent push mode If the remote is empty, we do now bother computing head changes and the 'pushbranchmap' attribute stays at None. We now handle and tests this case.

show: config option to register aliases for views As part of using `hg show` in my daily workflow, I've found it slightly annoying to have to type full view names, complete with a space. I've locally registered an alias for "swork = show work." I think others will have this same complaint and could benefit from some automation to streamline the creation of aliases. So, this commit introduces a config option that allows `hg show` views to be automatically aliased using a given prefix. e.g. a value of "s" will automatically register "swork" and "sbookmarks." Multiple values can be given for ultimate flexibility. This arguably isn't needed now. But since we don't register aliases if there will be a collision and we're bound to have a collision, it makes sense to allow multiple prefixes so specific views can avoid collisions by using different prefixes.