Fix bug caused by lack of local variables in Bourne Shell (yuck)

Signed-off-by: Robin Green <greenrd@greenrd.org>
export: Again, only check for annihilation if a branch is tgish

Signed-off-by: Robin Green <greenrd@greenrd.org>
Added 'tg checkout'.

This is a convenience command, that helps select the next topic branch
to work on.  You can navigate the dependency graph with 'tg checkout
push' and 'tg checkout pop', and select a specific branch with 'tg
checkout goto PATTERN'.

Using this command, you can give your topic branches descriptive
names, while not having to type those names repeatedly as you switch
branches.

Signed-off-by: Per Cederqvist <ceder@lysator.liu.se>
Signed-off-by: Robin Green <greenrd@greenrd.org>