add a "Git changes" segment to the prompt

[amtoine]

wait for

• test the Git prompt #122 (to include a test and make sure the changes appear in the prompt)

changelog

• refactor the short Git status into git/lib get-status
• adds a segment containing Git changes

details

• if there are no changes in the index, the prompt won't add anything.
• if there are changes, they will appear after the (branch:commit) segment inside [...]
• in order
  • staged changes will show up as one _
  • unstaged changes will show up as one !
  • untracked changes will show up as one ?