config: don't set source when no source is specified - don't overwrite with ''

This prevents ui.fixconfig from overwriting the source of paths and it will
thus show up in showconfig --debug.

mercurial/config.py

@@ -93,6 +93,7 b' class config(object):'
         if section not in self:
             self._data[section] = sortdict()
         self._data[section][item] = value
+        if source:
         self._source[(section, item)] = source
 
     def restore(self, data):

tests/test-hgrc.t

@@ -201,3 +201,14 b' plain mode with exceptions'
   --verbose: ui.verbose=False
   --debug: ui.debug=True
   --quiet: ui.quiet=False
+
+source of paths is not mangled
+
+  $ cat >> $HGRCPATH <<EOF
+  > [paths]
+  > foo = bar
+  > EOF
+  $ hg showconfig --debug paths
+  plain: True
+  read config from: $TESTTMP/hgrc
+  $TESTTMP/hgrc:17: paths.foo=$TESTTMP/bar