config: don't BUG when both kvi and source are set

config: don't BUG when both kvi and source are set

When iterating through config, we read config source metadata from
global values - either a "struct config_source + enum config_scope"
or a "struct key_value_info", using the current_config* functions. Prior
to the series starting from 0c60285147 (config.c: create config_reader
and the_reader, 2023-03-28), we weren't very picky about which values we
should read in which situation; we did note that both groups of values
generally shouldn't be set together, but if both were set,
current_config* preferentially reads key_value_info. When that series
added more structure, we enforced that either the former (when parsing a
config source) can be set, or the latter (when iterating a config set),
but *never* both at the same time. See 9828453ff0 (config.c: remove
current_config_kvi, 2023-03-28) and 5cdf18e7cd (config.c: remove
current_parsing_scope, 2023-03-28).

That was a good simplifying constraint that helped us reason about the
global state, but it turns out that there is at least one situation
where we need both to be set at the same time: in a blobless partial
clone where .gitmodules is missing. "git fetch" in such a repo will
start a config parse over .gitmodules (setting the config_source), and
Git will attempt to lazy-fetch it from the promisor remote. However,
when we try to read the promisor configuration, we start iterating a
config set (setting the key_value_info), and we BUG() out because that's
not allowed any more.

Teaching config_reader to gracefully handle this is somewhat
complicated, but fortunately, there are proposed changes to the config.c
machinery to get rid of this global state, and make the BUG() obsolete
[1]. We should rely on that as the eventual solution, and avoid doing
yet another refactor in the meantime.

Therefore, fix the bug by removing the BUG() check. We're reverting to
an older, less safe state, but that's generally okay since
key_value_info is always preferentially read, so we'd always read the
correct values when we iterate a config set in the middle of a config
parse (like we are here). The reverse would be wrong, but extremely
unlikely to happen since very few callers parse config without going
through a config set.

[1] https://lore.kernel.org/git/pull.1497.v3.git.git.1687290231.gitgitgadget@gmail.com

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>