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.

themes/agnoster.zsh-theme

@@ -29,6 +29,7 @@
 # A few utility functions to make it easy and re-usable to draw segmented prompts
 
 CURRENT_BG='NONE'
+zmodload zsh/parameter
 
 # Special Powerline characters
 
@@ -55,23 +56,23 @@ prompt_segment() {
   [[ -n $1 ]] && bg="%K{$1}" || bg="%k"
   [[ -n $2 ]] && fg="%F{$2}" || fg="%f"
   if [[ $CURRENT_BG != 'NONE' && $1 != $CURRENT_BG ]]; then
-    echo -n " %{$bg%F{$CURRENT_BG}%}$SEGMENT_SEPARATOR%{$fg%} "
+    PROMPT+=" %{$bg%F{$CURRENT_BG}%}$SEGMENT_SEPARATOR%{$fg%} "
   else
-    echo -n "%{$bg%}%{$fg%} "
+    PROMPT+="%{$bg%}%{$fg%} "
   fi
   CURRENT_BG=$1
-  [[ -n $3 ]] && echo -n $3
+  [[ -n $3 ]] && PROMPT+=$3
 }
 
 # End the prompt, closing any open segments
 prompt_end() {
   if [[ -n $CURRENT_BG ]]; then
-    echo -n " %{%k%F{$CURRENT_BG}%}$SEGMENT_SEPARATOR"
+    PROMPT+=" %{%k%F{$CURRENT_BG}%}$SEGMENT_SEPARATOR"
   else
-    echo -n "%{%k%}"
+    PROMPT+="%{%k%}"
   fi
-  echo -n "%{%f%}"
-  CURRENT_BG=''
+  PROMPT+="%{%f%}"
+  CURRENT_BG='NONE'
 }
 
 ### Prompt components
@@ -93,9 +94,9 @@ prompt_git() {
     PL_BRANCH_CHAR=$'\ue0a0'         # 
   }
   local ref dirty mode repo_path
-  repo_path=$(git rev-parse --git-dir 2>/dev/null)
 
   if $(git rev-parse --is-inside-work-tree >/dev/null 2>&1); then
+    repo_path=$(git rev-parse --git-dir 2>/dev/null)
     dirty=$(parse_git_dirty)
     ref=$(git symbolic-ref HEAD 2> /dev/null) || ref="➦ $(git rev-parse --short HEAD 2> /dev/null)"
     if [[ -n $dirty ]]; then
@@ -123,7 +124,7 @@ prompt_git() {
     zstyle ':vcs_info:*' formats ' %u%c'
     zstyle ':vcs_info:*' actionformats ' %u%c'
     vcs_info
-    echo -n "${ref/refs\/heads\//$PL_BRANCH_CHAR }${vcs_info_msg_0_%% }${mode}"
+    PROMPT+="${ref/refs\/heads\//$PL_BRANCH_CHAR }${vcs_info_msg_0_%% }${mode}"
   fi
 }
 
@@ -135,15 +136,15 @@ prompt_bzr() {
         revision=`bzr log | head -n2 | tail -n1 | sed 's/^revno: //'`
         if [[ $status_mod -gt 0 ]] ; then
             prompt_segment yellow black
-            echo -n "bzr@"$revision "✚ "
+            PROMPT+="bzr@$revision ✚ "
         else
             if [[ $status_all -gt 0 ]] ; then
                 prompt_segment yellow black
-                echo -n "bzr@"$revision
+                PROMPT+="bzr@$revision"
 
             else
                 prompt_segment green black
-                echo -n "bzr@"$revision
+                PROMPT+="bzr@$revision"
             fi
         fi
     fi
@@ -151,7 +152,7 @@ prompt_bzr() {
 
 prompt_hg() {
   (( $+commands[hg] )) || return
-  local rev status
+  local rev st branch
   if $(hg id >/dev/null 2>&1); then
     if $(hg prompt >/dev/null 2>&1); then
       if [[ $(hg prompt "{status|unknown}") = "?" ]]; then
@@ -166,7 +167,7 @@ prompt_hg() {
         # if working copy is clean
         prompt_segment green black
       fi
-      echo -n $(hg prompt "☿ {rev}@{branch}") $st
+      PROMPT+="$(hg prompt "☿ {rev}@{branch}")$st"
     else
       st=""
       rev=$(hg id -n 2>/dev/null | sed 's/[^-0-9]//g')
@@ -180,7 +181,7 @@ prompt_hg() {
       else
         prompt_segment green black
       fi
-      echo -n "☿ $rev@$branch" $st
+      PROMPT+="☿ $rev@$branch$st"
     fi
   fi
 }
@@ -207,7 +208,7 @@ prompt_status() {
   symbols=()
   [[ $RETVAL -ne 0 ]] && symbols+="%{%F{red}%}✘"
   [[ $UID -eq 0 ]] && symbols+="%{%F{yellow}%}⚡"
-  [[ $(jobs -l | wc -l) -gt 0 ]] && symbols+="%{%F{cyan}%}⚙"
+  [[ ${#jobstates} -ne 0 ]] && symbols+="%{%F{cyan}%}⚙"
 
   [[ -n "$symbols" ]] && prompt_segment black default "$symbols"
 }
@@ -215,6 +216,7 @@ prompt_status() {
 ## Main prompt
 build_prompt() {
   RETVAL=$?
+  PROMPT='%{%f%b%k%}'
   prompt_status
   prompt_virtualenv
   prompt_context
@@ -223,6 +225,8 @@ build_prompt() {
   prompt_bzr
   prompt_hg
   prompt_end
+  PROMPT+=' '
 }
 
-PROMPT='%{%f%b%k%}$(build_prompt) '
+autoload -U add-zsh-hook
+add-zsh-hook precmd build_prompt