test-subrepo-git.t on windows

[Eduard-Cristian Stefan]

Is it ok to test for Windows in subrepo.py and use 'git.cmd' instead of 'git'
in the subprocess.Popen call in _gitnodir?

Also, the output of the hg forget call will be updated.

--- d:\hg\tests\test-subrepo-git.t
+++ d:\hg\tests\test-subrepo-git.t.err
@@ -512,7 +512,7 @@
 Test forgetting files, not implemented in git subrepo, used to
 traceback
   $ hg forget 'notafile*'
-  notafile*: No such file or directory
+  notafile*: The filename, directory name, or volume label syntax is incorrect
   [1]

   $ cd ..

Eduard