pre-commit: use ^ anchors in `files`

Some `files` regexes use a ^ anchor, some don't.  I think it's good to
be explicit about it (and consistent), so use it everywhere.

Change-Id: I14eb2082a1c0ff6bc219e202fb16c3f12001fd73
Approved-By: Tom Tromey <tom@tromey.com>