--- a/mercurial/ui.py	Wed Aug 02 22:51:19 2017 +0900
+++ b/mercurial/ui.py	Sat Aug 05 13:19:09 2017 -0700
@@ -1215,6 +1215,7 @@
         # call write() so output goes through subclassed implementation
         # e.g. color extension on Windows
         self.write(prompt, prompt=True)
+        self.flush()
 
         # instead of trying to emulate raw_input, swap (self.fin,
         # self.fout) with (sys.stdin, sys.stdout)