Revert "avoid replacing "/home/user-alt" by "~-alt""

Doesn't work in home folder as $HOME and $PWD don't have any trailing "/".
This reverts commit 4a64c6ccec5efe49e2feb7a78dff556638088093.
bugfix #107: zsh-compatible if statement

An if statement was used without the conditional operator: if [[ "" ]]
This patch adds one: if [[ ! -z "" ]]
Making the code zsh compatible.
Replace lptag by prompt_tag

prompt_tag is more consistent with the other prompt_* functions.
 Issue #108 - Zsh:error in git dir if there is new files

It was a problem with local declaration and assignation in the same
time. Don't ask me why zsh complains, but it did.
Also, I took the liberty to make a similar change en bzr part, just a
precaution.
Merge pull request #111 from ldidry/108

 Issue #108 - Zsh:error in git dir if there is new files