[pre-commit] Fix codespell-log hook

A recent commit added this top-level setting to .pre-commit-config.yaml:
...
files: '^(gdb|gdbserver|gdbsupport)/'
...

This broke the codespell-log hook, which is a commit-msg hook, which is called
with the commit message as first argument, typically .git/COMMIT_EDITMSG.

However, the top-level files setting filters out .git/COMMIT_EDITMSG, with the
consequence that the commit-msg hook is no longer called.

It seems obvious to me that this is a pre-commit bug: the files field is there
to filter files in the repository, which .git/COMMIT_EDITMSG is not one of.
But upstream disagrees [1].

The fix suggested upstream is to include .git/COMMIT_EDITMSG in the default
files setting.

That indeed works for a regular commit, but not for something like this:
...
$ tmp=$(mktemp)
$ echo 'msg' > $tmp
$ pre-commit run --hook-stage commit-msg --commit-msg-filename $tmp
...
which is roughly what we're using in the regression test.

We can't use .git/COMMIT_EDITMSG in the regression test, because the user may
be editing it, or using it in some other way.

We also cannot use say gdb/testsuite/gdb.src/commit-msg.txt, because using
that filename doesn't detect the regression.

[1] https://github.com/pre-commit/pre-commit/issues/3720