hg.bat: return exit code explicitly for indirect invocation

When "hg.bat" is invoked via interactive shell "cmd.exe" on Windows,
it can store own exit code into ERRORLEVEL correctly, regardless of
explicit "exit" statement in it: "cmd.exe" seems to hold ERRORLEVEL
updated by the last command in the batch file (= "python hg", in
"hg.bat" case).

On the other hand, "hg.bat" is invoked indirectly via
"subprocess.Popen" (e.g. shell alias, hooks, hgclient and so on), the
parent process always receives exit code 0 from spawned "hg.bat":
batch files on Windows seem not to be really spawned like as shell
scripts on UNIX, but to be executed in the "cmd.exe" process.

This patch returns exit code explicitly for indirect invocation.

"/b" should be specified for "exit" to prevent "cmd.exe" from being
terminated when "hg.bat" is invoked interactively from it.

contrib/win32/hg.bat

@@ -15,3 +15,5 b' if exist "%~dp0..\\python.exe" ('
     python "%~dp0hg" %*
 )
 endlocal
+
+exit /b %ERRORLEVEL%

tests/test-alias.t

@@ -525,3 +525,12 b' return code of command and shell aliases'
   [1]
   $ hg exit1
   [1]
+
+#if no-outer-repo
+  $ hg root
+  abort: no repository found in '$TESTTMP' (.hg not found)!
+  [255]
+  $ hg --config alias.hgroot='!hg root' hgroot
+  abort: no repository found in '$TESTTMP' (.hg not found)!
+  [255]
+#endif