* agnoster: improve perf with use of $jobstates (zsh/parameter module)
This saves multiple fork/execs (for the subshell and wc) each
time the prompt is rendered
* agnoster: compute git repo_path only when in a git repo
this avoids needlessly invoking git twice every time we render the prompt
* agnoster: avoid subshell when rendering prompt
Rather than forking a subshell to print the prompt, construct it
incrementally by appending to $PROMPT.
v2: fix incorrect CURRENT_BG
v3: fix bzr and hg prompting
Thanks @mcornella for the help.