[clang] Improve clang-format-diff help output (PR #73491)

[Owen Pan via cfe-commits]

owenca wrote:

See below. Seems it's already working as expected without this patch.
```
$ clang-format -version
clang-format version 17.0.5
$ clang-format-diff.py foo
usage: clang-format-diff.py [-h] [-i] [-p NUM] [-regex PATTERN] [-iregex PATTERN]
                            [-sort-includes] [-v] [-style STYLE]
                            [-fallback-style FALLBACK_STYLE] [-binary BINARY]
clang-format-diff.py: error: unrecognized arguments: foo
$ ln -s clang-format-diff.py cfd
$ cfd foo
usage: cfd [-h] [-i] [-p NUM] [-regex PATTERN] [-iregex PATTERN] [-sort-includes] [-v]
           [-style STYLE] [-fallback-style FALLBACK_STYLE] [-binary BINARY]
cfd: error: unrecognized arguments: foo
```

https://github.com/llvm/llvm-project/pull/73491