Similar to the b C command to create a *new* branch named after a
clubhouse ticket, add a new b M command to *rename* a branch after the
currently clocked-in clubhouse ticket.
Change-Id: I750106f5d417517fd8114536d9dc0905380d616a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1970
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>