[clang] [Windows] Add git-clang-format wrapper bat file (PR #69228)

[Owen Pan via cfe-commits]

================
@@ -0,0 +1 @@
+py -3 git-clang-format %*
----------------
owenca wrote:

@llvm-beanz This doesn't seem to work if `git-clang-format` is run from another directory:
```
C:\Users\Owen\llvm-project>clang\tools\clang-format\git-clang-format

C:\Users\Owen\llvm-project>py -3 git-clang-format
C:\Users\Owen\AppData\Local\Programs\Python\Python39\python.exe: can't open file 'C:\Users\Owen\llvm-project\git-clang-format': [Errno 2] No such file or directory

C:\Users\Owen\llvm-project>
```

https://github.com/llvm/llvm-project/pull/69228