ui: make sure buffer is flushed before waiting for user input (issue5587)

Without this patch on Windows 'hg ci -i' hangs waiting for user input
and "examine changes to 'file'? [Ynesfdaq?]" is never displayed (at least
if the diff is sufficiently small). When Ctrl+C is pressed, this prompt
becomes visible, which suggests that the buffer just wasn't flushed.
I've never seen this happening on Linux, but this looks harmless enough
to not platform-gate it.

mercurial/ui.py

@@ -1215,6 +1215,7 b' class ui(object):'
         # 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)