Optimize _lp_shorten_path by using bash/zsh features

Optimizations:
- move variable declaration in blocks where they will be used
  (so they are not computed in blocks where they are not used)
- "local -i" for integer variables
- if [[ $(( ... )) -gt $(( ... )) ]]   ==>  if (( ... > ... ))
- i=$((i+1))   ==>  let i++
As a side effect the readability of the code is improved.
Merge branch 'optimize/shorten_path' of https://github.com/dolmen/liquidprompt into develop

Conflicts:
	liquidprompt