patch 7.4.737
Problem:    On MS-Windows vimgrep over arglist doesn't work (Issue 361)
Solution:   Only escape backslashes in ## expansion when it is not used as the
            path separator. (James McCoy)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index af63b6c..9ae8fff 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -10746,7 +10746,11 @@
 		}
 		for ( ; *p != NUL; ++p)
 		{
-		    if (*p == ' ' || *p == '\\')
+		    if (*p == ' '
+#ifndef BACKSLASH_IN_FILENAME
+			    || *p == '\\'
+#endif
+			    )
 		    {
 			/* insert a backslash */
 			if (retval != NULL)