update: add experimental config for default way of handling dirty wdir This allows the user to set e.g. experimental.updatecheck=abort to abort update if the working directory is dirty, but still be able to override the behavior with e.g. --merge when needed. I considered adding a --mergelinear option to get back the old behavior even when experimental.updatecheck=abort is set, but I couldn't see why anyone would prefer that over --merge. The default is read in hg.updatetotally(), which means it also applies to "hg pull -u" and "hg unbundle -u".