Support av stack branch <new branch> <base branch> (#265)

address #244

```sh
$ av stack branch new-branch parent-branch
```

cmd/av/stack_branch.go

@@ -28,17 +28,19 @@ var stackBranchFlags struct {
 	Force bool
 }
 var stackBranchCmd = &cobra.Command{
-	Use:     "branch [flags] <branch-name>",
+	Use:     "branch [flags] <branch-name> [<parent-branch>]",
 	Aliases: []string{"b", "br"},
 	Short:   "create a new stacked branch",
 	Long: `Create a new branch that is stacked on the current branch.
 
+<parent-branch>. If omitted, the new branch bases off the current branch.
+
 If the --rename/-m flag is given, the current branch is renamed to the name
 given as the first argument to the command. Branches should only be renamed
 with this command (not with git branch -m ...) because av needs to update
 internal tracking metadata that defines the order of branches within a stack.`,
 	SilenceUsage: true,
-	Args:         cobra.ExactArgs(1),
+	Args:         cobra.RangeArgs(1, 2),
 	RunE: func(cmd *cobra.Command, args []string) (reterr error) {
 		repo, err := getRepo()
 		if err != nil {
@@ -51,6 +53,10 @@ internal tracking metadata that defines the order of branches within a stack.`,
 		}
 
 		branchName := args[0]
+		if len(args) == 2 {
+			stackBranchFlags.Parent = args[1]
+		}
+
 		if stackBranchFlags.Rename {
 			return stackBranchMove(repo, db, branchName, stackBranchFlags.Force)
 		}