Revert "let's try this fancier __git_ps1 implementation"

This reverts commit c49c758bb2b0b4c19e2ae019bcc9406533735e52.
Revert "remove nonworking __git_ps1"

This reverts commit a89386ed85e8c117eab9cb00b710ec0e14a7b099.
run every command, not at PS1 instantiation only
working __git_ps1