cmd: Handle unechoed rem commands inside a (..) section.

When processing a (..) multiline section, each line is processed and
if it starts with a '@' it is not echoed, but more importantly if is
'rem' then anything else on that line should be ignored. The reported
issue was that a pipe was being executed when it was hidden behind a
rem, which was trigged by the preceeding '@' character not being
skipped.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=45729
Signed-off-by: Jason Edmeades <us@edmeades.me.uk>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

programs/cmd/tests/test_builtins.cmd

@@ -1427,6 +1427,12 @@ for /L %%i in (2,2,1) do (
   echo %%i
   echo FAILED
 )
+echo --- rems inside for loops
+for /f %%i IN ("hello") DO (
+   REM foo|echo ERROR unexpected execution 1
+   @REM foo|echo ERROR unexpected execution 2
+   @     REM foo|echo ERROR unexpected execution 3
+)
 echo --- ifs inside for loops
 for %%i in (test) do (
     echo a1

programs/cmd/tests/test_builtins.cmd.exp

@@ -996,6 +996,7 @@ ErrorLevel 0
 -1
 1
 3
+--- rems inside for loops
 --- ifs inside for loops
 a1
 b1

programs/cmd/wcmdmain.c

@@ -2308,12 +2308,21 @@ WCHAR *WCMD_ReadAndParseLine(const WCHAR *optionalcmd, CMD_LIST **output, HANDLE
 
         } while (*extraData == 0x00);
         curPos = extraSpace;
-        if (context) handleExpansion(extraSpace, FALSE, FALSE);
+
+        /* Skip preceding whitespace */
+        while (*curPos == ' ' || *curPos == '\t') curPos++;
+
+        /* Replace env vars if in a batch context */
+        if (context) handleExpansion(curPos, FALSE, FALSE);
+
         /* Continue to echo commands IF echo is on and in batch program */
-        if (context && echo_mode && extraSpace[0] && (extraSpace[0] != '@')) {
+        if (context && echo_mode && *curPos && *curPos != '@') {
           WCMD_output_asis(extraSpace);
           WCMD_output_asis(newlineW);
         }
+
+        /* Skip repeated 'no echo' characters and whitespace */
+        while (*curPos == '@' || *curPos == ' ' || *curPos == '\t') curPos++;
       }
     }