Added initial zsh completion support to git_index

zsh now completes all arguments and all directories inside $GIT_REPO_DIR. This is till buggy because not all directories in $GIT_REPO_DIR are actual git projects. Next update will include completing the list in the .git_index file.
Eliminated zsh references in completion

Since there is another, totally different completion function, this patch eliminates
all references to zsh in _git_index_tab_completion.
Merge branch 'master' of git://github.com/ndbroadbent/scm_breeze
Merge branch 'master' of git://github.com/ndbroadbent/scm_breeze
Merge branch 'master' of git://github.com/ndbroadbent/scm_breeze
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
Merge pull request #107 from g5pw/master

Add ZSH completion support