[git-clang-format] Do not apply clang-format to symlinks

This fixes PR46992.

Git stores symlinks as text files and we should not format them even if
they have one of the requested extensions.

(Move the call to `cd_to_toplevel()` up a few lines so we can also print
the skipped symlinks during verbose output.)

Differential Revision: https://reviews.llvm.org/D101878