combo-layer: let user choose where properties get updated

When updating a property (at the moment, only 'last_revision'), first check whether the user already added it to the main config. If so, update there even if there is a local config.

This way, 'last_revision' can be shared between developers as part of the repository while still configuring per-developer repo paths outside of the git repository in a local config.

An example of a repository which is set up like that is tizen-distro, with instructions for such a collaborative maintenance found here:
https://review.tizen.org/git/?p=scm/bb/tizen-distro.git;a=blob;f=README;hb=refs/heads/tizen-ivi

---
 scripts/combo-layer | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

index 19d64e6..489d180 100755
@@ -108,7 +108,9 @@ class Configuration(object):
                         readsection(self.localparser, section, repo)
 
     def update(self, repo, option, value, initmode=False):
-        if self.localparser:
+        # If the main config has the option already, that is what we
+        # are expected to modify.
+        if self.localparser and not self.parser.has_option(repo, option):
             parser = self.localparser
             section = "%s|%s" % (repo, self.combobranch)
             conffile = self.localconffile

2.1.4