little enhancements

[sclo]

First, thanks for this old pepite :)

Enhancements :

• use TASKBRANCH env variable if exists, else use default 'tasks' branch VS the fixed 'git-task' one
• use DEBUG env variable and modify all commented '#echo ' lines
• use 'git rev-parse --show-toplevel' to find the root git directory

I think this resolve 3 of the 5 ToDo/Issue on your 'git-task' branch ;)

PS : sorry for the aggressive reindent

Cya