Declare local variables

Since we are using the _arguments helper, we need to define some local
variables in order to avoid altering the global environment.
Change completion title

It makes more sense to name the category 'Git projects' instead of 'Project
name'.
Basic zsh completion for git_index

The script now completes all projects in the index and all files in GIT_PROJECTS
directory.
Complete commands when using batch-cmd
No need to have semicolons