help: move column config discovery to help.c library

help: move column config discovery to help.c library

When a git_config() call was added in dbfae689690 (help: reuse
print_columns() for help -a, 2012-04-13) to read the column config
we'd always use the resulting "colopts" variable.

Then in 63eae83f8f3 (help: add "-a --verbose" to list all commands
with synopsis, 2018-05-20) we started only using the "colopts" config
under "--all" if "--no-verbose" was also given, but the "git_config()"
call was not moved inside the "verbose" branch of the code.

This change effectively does that, we'll only call list_commands()
under "--all --no-verbose", so let's have it look up the config it
needs. See 26c7d067832 (help -a: improve and make --verbose default, 2018-09-29) for another case in help.c where we look up config.

The get_colopts() function is named for consistency with the existing
get_alias() function added in 26c7d067832.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>