procutil: don't allow the main 'hg' script to be treated as the Windows exe

Previously, there were a handful of errors like this:

     $ hg prefetch --repack
     (running background incremental repack)
  +  abort: %1 is not a valid Win32 application
  +  [255]

CreateProcess() doesn't append .exe when `lpApplicationName` contains a path,
and a python script isn't directly executable.

mercurial/utils/procutil.py

@@ -241,7 +241,7 b' def hgexecutable():'
                 _sethgexecutable(encoding.environ['EXECUTABLEPATH'])
             else:
                 _sethgexecutable(pycompat.sysexecutable)
-        elif (os.path.basename(
+        elif (not pycompat.iswindows and os.path.basename(
             pycompat.fsencode(getattr(mainmod, '__file__', ''))) == 'hg'):
             _sethgexecutable(pycompat.fsencode(mainmod.__file__))
         else: