[clang] [git-clang-format] add fallback style argument option (PR #137609)

[Owen Pan via cfe-commits]

================
@@ -189,6 +189,13 @@ def main():
         default=config.get("clangformat.style", None),
         help="passed to clang-format",
     ),
+    p.add_argument(
+        "--fallback-style",
+        help="The name of the predefined style used as a "
+        "fallback in case clang-format is invoked with "
+        "--style=file, but can not find the .clang-format "
+        "file to use.",
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owenca wrote:

```suggestion
        help="passed to clang-format",
```
to be consistent with `--style` above.

https://github.com/llvm/llvm-project/pull/137609