liquidprompt: change git diff counter behavior

Change the counter to include only the difference between
index and work tree, not between last commit and work tree
vim: small change in status bar
git: add alias for frequently used commands
screen: change appearance options

 * Remove from status line and caption unnecessary information
 * Change the colours in status line and caption
 * Change the colour of vertical split line to match that of
   horizontal split line
git: add options for diff command

 * Use mnemonic prefixes
 * Detect renames and copies