help: add additional information in the merge-tools section of config help The merge tool configuration in general has always been a confusing topic for user. We add mention of: - possible pre-existing configuration, - way to look at it, - the more detailed help topic about how merge-tools configuration works. This should help users find they way.

subrepo: use subrepo shortid method to generate subrepo diverged promptchoice We were always using only the first 12 characters of the subrepo revision id when generating the "subrepo diverged" promptchoice. This is not necessarily correct for non mercurial subrepos.