Add explicit option --repo for request-branch
This patch fixes a similar problem introduced to request-repo. A command
option is added for convenience. The global --name still works. So, when
packagers read request-branch help text with `fedpkg request-branch -h`,
they can easily know how to specify repository name directly without
additional step to look up global option --name.
Fixes #244
Signed-off-by: Chenxiong Qi <cqi@redhat.com>