[clang-format] Fix a bug in git-clang-format.bat (#75268)

Pass the fully-qualified path name (less the file extension) of
git-clang-format.bat to py so that it can be run from anywhere.

Fixes #75265.

Author: owenca

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

@@ -1 +1 @@
-py -3 git-clang-format %*
+py -3 %~pn0 %*