git-prompt.sh: strip unnecessary space in prompt string

Nobody has branch names that end with + or *.  Then why put a space
after the branch name and before [*|+][=|<|>] in the prompt string?

Before this, your prompt might have looked like:

    artagnon|master *=:~/src/git$

Now, it will look like:

    artagnon|master*=:~/src/git$

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
git-prompt.zsh: introduce thin ZSH wrapper

To facilitate a colored prompt in ZSH, write a thin wrapper around
git-prompt.sh, factoring out and overriding the coloring logic.  Since
ZSH lacks a PROMPT_COMMAND, instruct the user to execute __git_ps1
inside precmd().

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>