dispatch: defer environment variable resolution in alias commands (BC) Before this patch, if there are environment variables in an alias command, they will be expanded immediately when we first see the alias. This will cause issues with chg, because environment variable updates will not propagate to expanded arguments. This patch makes "args" of "cmdalias" a property that will be calculated every time when accessed.

rollback: add a config knob for entirely disabling the command This is of pretty high value for organizations that used to use p4 (as an example), since `p4 rollback` is what we call `hg backout`.

templater: add separate() template function A pretty common pattern in templates is adding conditional separators like so: {node}{if(bookmarks, " {bookmarks}")}{if(tags, " {tags}")} With this patch, the above can be simplified to: {separate(" ", node, bookmarks, tags)} The function is similar to the already existing join(), but with a few differences: * separate() skips empty arguments * join() expects a single list argument, while separate() expects each item as a separate argument * separate() takes the separator first in order to allow a variable number of arguments after it