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()".

mercurial/config.py

@@ -44,6 +44,7 b' class config(object):'
     def __init__(self, data=None):
         self._data = {}
         self._source = {}
+        self._unset = []
         if data:
             for k in data._data:
                 self._data[k] = data[k].copy()
@@ -58,6 +59,10 b' class config(object):'
         for d in self.sections():
             yield d
     def update(self, src):
+        for s, n in src._unset:
+            if s in self and n in self._data[s]:
+                del self._data[s][n]
+                del self._source[(s, n)]
         for s in src:
             if s not in self:
                 self._data[s] = sortdict()
@@ -173,6 +178,7 b' class config(object):'
                     continue
                 if self.get(section, name) is not None:
                     del self._data[section][name]
+                self._unset.append((section, name))
                 continue
 
             raise error.ParseError(l.rstrip(), ("%s:%s" % (src, line)))

tests/test-config.t

@@ -11,3 +11,34 b' Test case sensitive configuration'
   Section.KeY=Case Sensitive
   Section.key=lower case
 
+Test "%unset"
+
+  $ cat >> $HGRCPATH <<EOF
+  > [unsettest]
+  > local-hgrcpath = should be unset (HGRCPATH)
+  > %unset local-hgrcpath
+  > 
+  > global = should be unset (HGRCPATH)
+  > 
+  > both = should be unset (HGRCPATH)
+  > 
+  > set-after-unset = should be unset (HGRCPATH)
+  > EOF
+
+  $ cat >> .hg/hgrc <<EOF
+  > [unsettest]
+  > local-hgrc = should be unset (.hg/hgrc)
+  > %unset local-hgrc
+  > 
+  > %unset global
+  > 
+  > both = should be unset (.hg/hgrc)
+  > %unset both
+  > 
+  > set-after-unset = should be unset (.hg/hgrc)
+  > %unset set-after-unset
+  > set-after-unset = should be set (.hg/hgrc)
+  > EOF
+
+  $ hg showconfig unsettest
+  unsettest.set-after-unset=should be set (.hg/hgrc)