IOTBTOOL-279: Fix default branch

Git now uses the branch that the cache was checked out to, rather than master, by default.
This then follows the selection of 'default branch' as understood by github

Author: mark-edgeworth

mbed/mbed.py

@@ -1278,12 +1278,23 @@ def clone(self, url, path, rev=None, depth=None, protocol=None, offline=False, *
                     with self.cache_lock_held(url):
                         shutil.copytree(cache, path)
 
+                    #
+                    # If no revision was specified, use the branch associated with the cache. In the
+                    # github case this will be the default branch (IOTBTOOL-279)
+                    #
+                    if not rev:
+                        with cd(cache):
+                            branch = scm.getbranch()
+                            if branch:
+                                rev = branch
+                            else:
+                                # Can't find the cache branch; use a sensible default
+                                rev = scm.default_branch
+
                     with cd(path):
                         scm.seturl(formaturl(url, protocol))
                         scm.cleanup()
                         info("Update cached copy from remote repository")
-                        if not rev:
-                            rev = scm.default_branch
                         scm.update(rev, True, is_local=offline)
                         main = False
                 except (ProcessException, IOError):