Unify alert and merge tool configuration !!! This changes the autosync command interface !!! KISS. Now there's only one way to set an alerter tool, using the option alert in section autosync of an HGRC file. Same thing for the merge tool: it is defined in autosync->merge. This benefit here is that this clearly distinguishes the merge tool configuration for the autosync command and ohter commands which use ui->merge or HGMERGE to get the merge tool. This change also fixes a parameter type error when calling the alert tool.