ab/config-based-hooks-2

> * ab/config-based-hooks-2 (2021-10-15) 14 commits
>  - run-command: remove old run_hook_{le,ve}() hook API
>  - receive-pack: convert push-to-checkout hook to hook.h
>  - read-cache: convert post-index-change to use hook.h
>  - commit: convert {pre-commit,prepare-commit-msg} hook to hook.h
>  - git-p4: use 'git hook' to run hooks
>  - send-email: use 'git hook run' for 'sendemail-validate'
>  - git hook run: add an --ignore-missing flag
>  - merge: convert post-merge to use hook.h
>  - hooks: convert 'post-checkout' hook to hook library
>  - am: convert applypatch to use hook.h
>  - rebase: convert pre-rebase to use hook.h
>  - gc: use hook library for pre-auto-gc hook
>  - hook: add 'run' subcommand
>  - Merge branch 'ab/config-based-hooks-1' into ab/config-based-hooks-2
>
>  More "config-based hooks".
>
>  Breaks t2400.