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()".