config: discard "%unset" values defined in the other files read in previously Before this patch, "%unset" can't unset values defined in the other files read in previously, even though online help document says that it can. It can unset only values defined in the same configuration file. For example, the value defined in "~/.hgrc" can't be unset by "%unset" in ".hg/hgrc" of the repository. This patch records "%unset"-ed values in "config.parse()", and discards corresponding values in "config.update()".