[clang] [clang-format] Fix a bug in `git-clang-format --binary` (PR #74176)

[Youngsuk Kim via cfe-commits]

JOE1994 wrote:

I've been using `git clang-format` without the `--bin` flag using the following setting

1. `PATH=$PATH:~/llvm-project/build/bin:~/llvm-project/clang/tools/clang-format`
2. Run `git clang-format HEAD~1` at `~/llvm-project` directory

After this revision, I get the following error:
```
error: cannot find executable "<FULL_PATH_UP_TO_LLVM_PROJECT>/llvm-project/clang-format"
```
(**<FULL_PATH_UP_TO_LLVM_PROJECT>** = full path to the parent directory of `llvm-project` directory)

https://github.com/llvm/llvm-project/pull/74176