cmake: determine list of extra built-ins dynamically

cmake: determine list of extra built-ins dynamically

In 0a21d0e08902 (Makefile: mark git-maintenance as a builtin,
2020-12-01), we marked git-maintenance as a builtin in the Makefile, but
forgot to do the same in `CMakeLists.txt`.

Rather than always play catch-up and adjust `git_builtin_extra`
manually, use the `BUILT_INS` definitions in the Makefile as
authoritative source and generate `git_builtin_extra` dynamically.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>