Add prompt config variable for async git prompt.

Setting `prompt_overrideGitStatusOptIn = true` will override the
`cmder.status` and `cmder.cmdstatus` git config settings and run the git
prompt status commands in the background.  But it only takes effect when
using Clink v1.2.10, since that's required in order to run prompt update
commands in the background.

vendor/clink.lua

@@ -74,9 +74,9 @@ 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
+    if prompt_overrideGitStatusOptIn then
+        cmderForceAsyncGitStatus = true
+    end
 else
     io_popenyield = io.popen
     clink_promptcoroutine = function (func)