[clang-format] git-clang-format --staged should format the index

When --staged (or --cached) use the index for formatting as well, not just
for the line numbers to format. Without this change git-clang-format gets
the changed line numbers based on the index, but then formats these lines on
the working tree version of the file.

This is a problem when the working tree and index differ. One common case
would be (and is the motivation behind this patch) when applying the
suggested changes git-clang-format --staged, then forgetting to add the
applied changes. When git-clang-format --staged --diff is used in a
pre-commit hook in this scenario, then the hook would allow committing the
improperly formatted changes, as the file is correctly formatted in the work
tree.

Fixes #56797.

Differential Revision: https://reviews.llvm.org/D130108