Add support for url.*.insteadof

Teach repo how to resolve URLs using the url.insteadof feature
that C Git natively uses during clone, fetch or push. This will
later allow repo to resolve a URL before accessing it directly.
We do not want to pre-resolve things and store the resolved URL
into individual projects, as this makes it impossible for the
user to undo the insteadof mapping at a later date.

Change-Id: I0f62e811197c53fbc8a8be424e3cabf4ed07b4cb
Signed-off-by: Shawn O. Pearce <sop@google.com>

git_config.py

@@ -198,6 +198,15 @@ class GitConfig(object):
     except KeyError:
       return False
 
+  def UrlInsteadOf(self, url):
+    """Resolve any url.*.insteadof references.
+    """
+    for new_url in self.GetSubSections('url'):
+      old_url = self.GetString('url.%s.insteadof' % new_url)
+      if old_url is not None and url.startswith(old_url):
+        return new_url + url[len(old_url):]
+    return url
+
   @property
   def _sections(self):
     d = self._section_dict