Remove status check on the SDK repo and add version check

If Appveyor fails to unzip, .git would be in an arbitrary state anyway.
Thus this check doesn't help. For the version check - rely on the branch
naming scheme in the SDK.

appveyor/build.bat

@@ -17,12 +17,8 @@ if not exist "%PHP_BUILD_CACHE_SDK_DIR%" (
 	echo Cloning remote SDK repository
 	git clone --branch %SDK_BRANCH% %SDK_REMOTE% "%PHP_BUILD_CACHE_SDK_DIR%" 2>&1 
 ) else (
-	git --git-dir="%PHP_BUILD_CACHE_SDK_DIR%\.git" --work-tree="%PHP_BUILD_CACHE_SDK_DIR%" status 2>&1
-	if %errorlevel% neq 0 (
-		rmdir /s /q "%PHP_BUILD_CACHE_SDK_DIR%"
-		echo Cloning remote SDK repository
-		git clone --branch %SDK_BRANCH% %SDK_REMOTE% "%PHP_BUILD_CACHE_SDK_DIR%" 2>&1 
-	) else (
+	for /f "tokens=*" %%a in ('type %PHP_BUILD_CACHE_SDK_DIR%\VERSION') do set GOT_SDK_VER=%%a
+	if "%GOT_SDK_VER%" NEQ "%SDK_BRANCH:~8%" (
 		echo Fetching remote SDK repository
 		git --git-dir="%PHP_BUILD_CACHE_SDK_DIR%\.git" --work-tree="%PHP_BUILD_CACHE_SDK_DIR%" fetch --prune origin 2>&1
 		echo Checkout SDK repository branch