Rudimentary support for multiple development branches

 stg branch

   show the name of the current branch.

 stg branch <branch-name>

   switch to the named branch.  after the switch is complete,
   all currently applied patches in the target branch are
   applied.

 stg branch --create <branch-name> [ <commit-sha1> ]

   create a new branch.  if <commit-sha1> is not specified then
   the branch is created starting at the commit pointed to by
   the current HEAD.  otherwise, the branch is created starting
   at the given commit.

   when the branch creation completes, the new branch becomes
   the current branch.

 stg branch --delete [ <branch-name> ]

   delete a branch and all patches it contains.  if no name is
   specified, delete the current branch and switch to the
   "master" branch.  otherwise, delete the named branch.

 stg branch --list

   list all the development branches in the current repository
   along with their descriptions and status.

 stg branch --rename <from-branch-name> <to-branch-name>

   rename a branch, and switch to the new name.

 stg branch -h

   print a synopsis of the "stg branch" command.

Signed-off-by: Chuck Lever <cel@netapp.com>