builtin/init: stop modifying `is_bare_repository_cfg`

builtin/init: stop modifying `is_bare_repository_cfg`

We're modifying `is_bare_repository_cfg` in "builtin/init.c" to indicate
whether the newly created repository is supposed to be a bare repository
or not.

This is ultimately unnecessary though: when initializing the repository
in `init_db()` we eventually set `is_bare_repository_cfg = !work_tree`,
so all that matters is whether or not we have a working tree configured,
and the working tree is set up in the non-bare in "builtin/init.c".

Stop modifying the global variable in "builtin/init.c" in favor of a
local variable.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>