split out the async prompt code

lib/async-git-prompt.zsh

@@ -0,0 +1,33 @@
+ASYNC_PROC=0
+function precmd() {
+    function async() {
+        # save to temp file
+        printf "%s" "$(git_prompt_info)" > "${HOME}/.zsh_tmp_prompt"
+
+        # signal parent
+        kill -s USR1 $$
+    }
+
+    # do not clear RPROMPT, let it persist
+
+    # kill child if necessary
+    if [[ "${ASYNC_PROC}" != 0 ]]; then
+        kill -s HUP $ASYNC_PROC >/dev/null 2>&1 || :
+    fi
+
+    # start background computation
+    async &!
+    ASYNC_PROC=$!
+}
+
+function TRAPUSR1() {
+    # read from temp file
+    PROMPT="${PROMPT_START} $(cat ${HOME}/.zsh_tmp_prompt) ${PROMPT_END}"
+
+    # reset proc number
+    ASYNC_PROC=0
+
+    # redisplay
+    zle && zle reset-prompt
+}
+