[clang-format] Fix path expansion inside git-clang-format.bat (#114078)

The trampoline script used on Windows (due to the absence of shebang
support) doesn't properly expand the path to the Python script, as it
leaves out the drive letter.

Functionally equivalent reproducer in action
```
PS C:\Users\mate> gc (gcm git-clang-formatish.bat).Source
@echo OFF
echo "%~pn0" %*
PS C:\Users\mate> git-clang-formatish
"\Users\mate\git-clang-formatish"
```

Adding `d` to the variable modifiers [as per the
docs](https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/for)
the drive letter is added. Even in the magical cases when it works.

(I couldn't reproduce, but I suspect it's only tested from some
bash/cygwin variant, where the path becomes `/c/Program Files/...`, but
the drive letter is needed. Without it, I also observed cases when used
via `git clang-format` (without the inital dash) it tries to infer the
drive letter based on the current working directory. In that case it
fails to find `D:\Program Files\LLVM\bin\clang-format.exe`, which
naturally fails, because `Program Files` is on `C:`)

Author: MathiasMagnus

clang/tools/clang-format/git-clang-format.bat

@@ -1 +1 @@
-py -3 "%~pn0" %*
+py -3 "%~dpn0" %*