Re-run Windows tests when they fail at least once

run-tests.bat

@@ -10,7 +10,21 @@ REM Use the first argument for selecting tests to run.
 if not "%1"=="" set tests=%1
 
 set VADER_OUTPUT_FILE=%~dp0\vader_output
+REM Automatically re-run Windows tests, which can fail some times.
+set tries=0
+
+RUN_TESTS:
+set /a tries=%tries%+1
 type nul > "%VADER_OUTPUT_FILE%"
 C:\vim\vim\vim80\vim.exe -u test/vimrc "+Vader! %tests%"
+set code=%ERRORLEVEL%
+
+IF %code% EQU 0 GOTO :SHOW_RESULTS
+IF %tries%  GEQ 2 GOTO :SHOW_RESULTS
+GOTO :RUN_TESTS
+
+SHOW_RESULTS:
 type "%VADER_OUTPUT_FILE%"
 del "%VADER_OUTPUT_FILE%"
+
+exit /B %code%