[clang] [clang-format][NFC] Use `prog` in clang-format-diff.py (PR #74399)

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darker --check --diff -r 192439db6e3fcccf98c850bda1b970a11c590bbb..ee039e7c50751fabdbaadae73a0a09bc905620f2 clang/tools/clang-format/clang-format-diff.py
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View the diff from darker here.
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``````````diff
--- clang-format-diff.py	2023-12-05 01:33:47.000000 +0000
+++ clang-format-diff.py	2023-12-05 01:40:08.802548 +0000
@@ -36,11 +36,11 @@
     from io import BytesIO as StringIO
 
 
 def main():
     parser = argparse.ArgumentParser(
-        description=__doc__.format(clang_format_diff='%(prog)s'),
+        description=__doc__.format(clang_format_diff="%(prog)s"),
         formatter_class=argparse.RawDescriptionHelpFormatter,
     )
     parser.add_argument(
         "-i",
         action="store_true",

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https://github.com/llvm/llvm-project/pull/74399