Use Clink async prompt filtering for git.

`git status` and `git diff` can be slow in large repos.  Clink v1.2.10
and higher support using Lua coroutines to do expensive parts of prompt
filtering in the background.  When the expensive parts complete, the
prompt gets refreshed.

This means even large repos can have fast prompts PLUS git status all
the time!

This change should be backward/forward compatible with both older and
newer versions of Clink (of course only newer versions will gain the
benefit).

vendor/clink.lua

@@ -63,6 +63,28 @@ local function get_folder_name(path)
 end
 
 
+---
+-- Forward/backward compatibility for Clink asynchronous prompt filtering.
+-- With Clink v1.2.10 and higher this lets git status run in the background and
+-- refresh the prompt when it finishes, to eliminate waits in large git repos.
+---
+local io_popenyield
+local clink_promptcoroutine
+local cached_info = {}
+if clink.promptcoroutine and io.popenyield then
+    io_popenyield = io.popenyield
+    clink_promptcoroutine = clink.promptcoroutine
+    -- Uncommenting this will override the cmderGitStatusOptIn and always show
+    -- git status when Clink is able to run it in the background.
+    --cmderForceAsyncGitStatus = true
+else
+    io_popenyield = io.popen
+    clink_promptcoroutine = function (func)
+        return func(false)
+    end
+end
+
+
 ---
 -- Setting the prompt in clink means that commands which rewrite the prompt do
 -- not destroy our own prompt. It also means that started cmds (or batch files
@@ -308,7 +330,7 @@ end
 -- @return {bool}
 ---
 local function get_git_status()
-    local file = io.popen("git --no-optional-locks status --porcelain 2>nul")
+    local file = io_popenyield("git --no-optional-locks status --porcelain 2>nul")
     for line in file:lines() do
         file:close()
         return false
@@ -323,7 +345,7 @@ end
 -- @return {bool} indicating true for conflict, false for no conflicts
 ---
 function get_git_conflict()
-    local file = io.popen("git diff --name-only --diff-filter=U 2>nul")
+    local file = io_popenyield("git diff --name-only --diff-filter=U 2>nul")
     for line in file:lines() do
         file:close()
         return true;
@@ -363,6 +385,22 @@ local function get_svn_status()
     return true
 end
 
+---
+-- Use a prompt coroutine to get git status in the background.
+-- Cache the info so we can reuse it next time to reduce flicker.
+---
+local function get_git_info_table()
+    local info = clink_promptcoroutine(function ()
+        return { status=get_git_status(), conflict=get_git_conflict() }
+    end)
+    if not info then
+        info = cached_info.git_info or {}
+    else
+        cached_info.git_info = info
+    end
+    return info
+end
+
 ---
 -- Get the status of working dir
 -- @return {bool}
@@ -402,19 +440,30 @@ local function git_prompt_filter()
 
     local git_dir = get_git_dir()
     local color
-    cmderGitStatusOptIn = get_git_status_setting()
+    cmderGitStatusOptIn = cmderForceAsyncGitStatus or get_git_status_setting()
     if git_dir then
         local branch = get_git_branch(git_dir)
         if branch then
+            -- If in a different repo or branch than last time, discard cached info.
+            if cached_info.git_dir ~= git_dir or cached_info.git_branch ~= branch then
+                cached_info.git_info = nil
+                cached_info.git_dir = git_dir
+                cached_info.git_branch = branch
+            end
+            -- Use git status if allowed.
             if cmderGitStatusOptIn then
                 -- if we're inside of git repo then try to detect current branch
                 -- Has branch => therefore it is a git folder, now figure out status
-                local gitStatus = get_git_status()
-                local gitConflict = get_git_conflict()
+                local gitInfo = get_git_info_table()
+                local gitStatus = gitInfo.status
+                local gitConflict = gitInfo.conflict
 
-                color = colors.dirty
-                if gitStatus then
+                if gitStatus == nil then
+                    color = colors.nostatus
+                elseif gitStatus then
                     color = colors.clean
+                else
+                    color = colors.dirty
                 end
 
                 if gitConflict then