config: introduce set_or_die wrappers

config: introduce set_or_die wrappers

A lot of call-sites for the existing family of `git_config_set`
functions do not check for errors that may occur, e.g. when the
configuration file is locked. In many cases we simply want to die
when such a situation arises.

Introduce wrappers that will cause the program to die in those
cases. These wrappers are temporary only to ease the transition
to let `git_config_set` die by default. They will be removed
later on when `git_config_set` itself has been replaced by
`git_config_set_gently`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>