agnoster: cut down on fork/execs improve performance when not in a repo (#6210)

* 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.