ui: suggest config --edit when no username is set

config: add example config file when -e called with no config

config: add initial implementation of --edit This launches the preferred editor on either: a) the first non-empty user rc file found b) the first user rc file in the search list

revert: extract actual revert in its own function With this small refactoring, the computation of the action to perform and the actual change to the disk happen in different function. This allows extension to wrap the later in case of need. The initial motivation is the `remotefilelog` extension that need to prefetch all the file content to be checked out.

config: move showconfig code and add config as primary alias Preparation for adding config --edit to launch an editor

revset: added _hexlist method to replace _list for %ln Now %ln expression goes through _hexlist and doesn't do any unnecesary processing anymore.