[bash] git_ahead_behind: don't calculate this if no remote isset
[bash] git_ahead_behind: print 0 ahead and 0 behind if no remote exist
[bash] avoid extra escaping of $Color* variables

This was need to inject this colors into PS1, but in next patches I will
make PS1 generation simpler, and there is no need in this.
[bash] custom-prompt-git: move bash code into helper function.

This will make changing of colored prompt for git more simpler.
[bash] custom-prompt-git: if we have uncommited changes also write in red