If the .git/hooks/pre-commit already exists, only add the clang-format line
if it doesn't already exist.

If clang-format isn't found, there is nothing to write to the file.

Continuation of commit 4e47b053c9d799.

All unittests still pass.

It should be --extensions "cpp,h,hpp,c"; previously, I naively tested that
it didn't bother .json files, but I not if it'll actually trigger for e.g.
a .h file...

Previously the clang-format.sh git hook script didn't specify the file
extensions to format; however it looks like with recent(?) clang-format
versions 'git clang-format' will consider .json files if such files were
changed by a commit, which makes the script fail, see:
https://invent.kde.org/frameworks/extra-cmake-modules/-/merge_requests/254

Sepcify the file extensions by passing --extensions to 'git clang-format'
command.

Also add ".hpp" to the file extensions we format, and use the same list of
file extensions everywhere.

If we are not tracking a specific branch, assume the remote is "origin"
which is git's default.

This fixes KDEInstallDirsTest.relative_or_absolute_qt unittest.

Otherwise, commits will be rejected because the hook cannot be found from the workdir root dir (there is only one .git directory in a checkout, but commands are run relative to the worktree root dir).