POLYGLOT_SHOW_UNTRACKED; bug fixes

• Git status calculation for untracked files may now be turned off by setting POLYGLOT_SHOW_UNTRACKED=0. Doing so has the effect of speeding up the prompt display considerably when one is working with extraordinarily large repositories.
• dash is now correctly recognized when it is a login shell.
• The @ sign was not displaying between the username and the hostname in ZSH. This has been corrected.