git: optimize changed lines extraction
git: optimize changed lines extraction

- use "git diff --shortstat" to replace 3 different calls to "git diff"
- drop usage of awk

tocomo/liquidprompt,colinlieberman/liquidprompt,enrdstrm/liquidprompt,illwieckz/liquidprompt,dpilone/liquidprompt,joshuarubin/dotfiles,milouse/liquidprompt,joshuarubin/dotfiles,tkuminecz/liquidprompt,joshuarubin/dotfiles,timhyson/liquidprompt,dan-cleinmark/liquidprompt,joshuarubin/dotfiles,elementalvoid/liquidprompt,laurentbardin/liquidprompt,BobHood/liquidprompt,lyoshenka/liquidprompt,kenrachynski/liquidprompt,alexprengere/liquidprompt,tstone2077/liquidprompt,tsbarnes/liquidprompt,jonathanjsimon/liquidprompt,nojhan/liquidprompt,smadhueagle/liquidprompt,mathieumd/liquidprompt,jaesivsm/liquidprompt,dtschan/liquidprompt,pkkolos/liquidprompt