Update git_head_status() with more checks and details
Update git_head_status() with more checks and details

Copy some checks from the git-prompt.sh script shipped with the git
completion scripts. Also provide details on how far along in a rebase
the user is.

pkkolos/liquidprompt,laurentbardin/liquidprompt,nojhan/liquidprompt,milouse/liquidprompt,jaesivsm/liquidprompt