Don't prompt the user for name/email unless necessary

If the user has already configured a workspace, use these values
when re-running 'repo init'.

Otherwise, if the user has global name and e-mail set, use these.

It's always possible to override this and be prompted by specifying
--config-name when running 'repo init'.

Change-Id: If45f0e4b14884071439fb02709dc5cb53f070f60
diff --git a/repo b/repo
index 1c71f07..0b20a9b 100755
--- a/repo
+++ b/repo
@@ -139,6 +139,11 @@
                  dest='no_repo_verify', action='store_true',
                  help='do not verify repo source code')
 
+# Other
+group = init_optparse.add_option_group('Other options')
+group.add_option('--config-name',
+                 dest='config_name', action="store_true", default=False,
+                 help='Always prompt for name/e-mail')
 
 class CloneFailure(Exception):
   """Indicate the remote clone of repo itself failed.