Ensure backslashes in windows filename (#1555)

cmd.exe doesn't work with '/' as path separator.

Co-authored-by: Mark Waite <mark.earl.waite@gmail.com>

src/main/java/jenkins/plugins/git/GitUsernamePasswordBinding.java

@@ -193,7 +193,8 @@ private String windowsArgEncodeFileName(String filename) {
             if (filename.contains("\"")) {
                 filename = filename.replace("\"", "^\"");
             }
-            return "\"" + filename + "\"";
+            /* Ensure backslashes since cmd doesn't work with slashes as path separators */
+            return "\"" + filename.replace("/", "\\") + "\"";
         }
     }