cmdutil: hide child window created by win32 spawndetached()

Hiding the child process window is not strictly necessary but it avoids opening
an empty shell window when running hg serve as well as a task in the task bar.
The window is hidden after the process is already started causing a single
flicker.

mercurial/cmdutil.py

@@ -613,6 +613,7 b' def service(opts, parentfn=None, initfn='
         except AttributeError:
             pass
         os.unlink(lockpath)
+        util.hidewindow()
         sys.stdout.flush()
         sys.stderr.flush()
 

mercurial/util.py

@@ -529,6 +529,14 b' else:'
 def lookup_reg(key, name=None, scope=None):
     return None
 
+def hidewindow():
+    """Hide current shell window.
+
+    Used to hide the window opened when starting asynchronous
+    child process under Windows, unneeded on other systems.
+    """
+    pass
+
 if os.name == 'nt':
     from windows import *
 else:

mercurial/win32.py

@@ -16,7 +16,7 b' used.'
 import win32api
 
 import errno, os, sys, pywintypes, win32con, win32file, win32process
-import winerror
+import winerror, win32gui
 import osutil, encoding
 from win32com.shell import shell, shellcon
 
@@ -172,3 +172,12 b' def set_signal_handler_win32():'
         win32process.ExitProcess(1)
     win32api.SetConsoleCtrlHandler(handler)
 
+def hidewindow():
+    def callback(*args, **kwargs):
+        hwnd, pid = args
+        wpid = win32process.GetWindowThreadProcessId(hwnd)[1]
+        if pid == wpid:
+            win32gui.ShowWindow(hwnd, win32con.SW_HIDE)
+
+    pid =  win32process.GetCurrentProcessId()
+    win32gui.EnumWindows(callback, pid)