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()".
Before this patch, there is no test script testing configuration
handling generally. "test-config-case.t" seems to be specific for
testing case sensitive configuration.
This patch renames from "test-config-case.t" to "test-config.t" for
centralization of tests around configuration handling.
