bitbake/fetch: add try_premirror method and special case git fetcher

Add a new method, try_premirror, to the Fetch object which checks to see
whether the file needs to be fetched from a premirror.
Override this in the Git fetcher to only require a pre-mirror fetch when the
clone directory does not exist.

Fixes [BUGID 290]

Signed-off-by: Joshua Lock <josh@linux.intel.com>

bitbake/lib/bb/fetch/git.py

@@ -90,6 +90,12 @@ class Git(Fetch):
             return True
         return False
 
+    def try_premirror(self, d, ud):
+        if os.path.exists(ud.clonedir):
+            return False
+
+        return True
+
     def go(self, loc, ud, d):
         """Fetch url"""