subrepo: support Git being named "git.cmd" on Windows (issue3173)

Popen does not consider "foo.cmd" equivalent to "foo" on Windows.
Unfortunately, the default MSYS Git installation installs only "git.cmd" into
the path by default.  This patch probes for both possible names on Windows.

mercurial/subrepo.py

@@ -850,6 +850,13 b' class gitsubrepo(abstractsubrepo):'
         self._ensuregit()
 
     def _ensuregit(self):
+        try:
+            self._gitexecutable = 'git'
+            out, err = self._gitnodir(['--version'])
+        except OSError, e:
+            if e.errno != 2 or os.name != 'nt':
+                raise
+            self._gitexecutable = 'git.cmd'
         out, err = self._gitnodir(['--version'])
         m = re.search(r'^git version (\d+)\.(\d+)\.(\d+)', out)
         if not m:
@@ -883,8 +890,8 b' class gitsubrepo(abstractsubrepo):'
         errpipe = None
         if self._ui.quiet:
             errpipe = open(os.devnull, 'w')
-        p = subprocess.Popen(['git'] + commands, bufsize=-1, cwd=cwd, env=env,
-                             close_fds=util.closefds,
+        p = subprocess.Popen([self._gitexecutable] + commands, bufsize=-1,
+                             cwd=cwd, env=env, close_fds=util.closefds,
                              stdout=subprocess.PIPE, stderr=errpipe)
         if stream:
             return p.stdout, None