[bash] add git_ahead_behind() helper
[bash] use git_ahead_behind() for bash prompt git additional

Now prompt can look like this:
(master)[1|0]$

First number - ahead
Second number - behind
[bash] git-prompt: trim behind commits:

Now it can look like this:
(master)[2]

Only ahead commits are showed, because there is no behind commits, and I
think that better to suppress them in output in this case.