docs: sort configuration variable groupings alphabetically
By and large, variable groupings in Documentation/config.txt are sorted alphabetically, though a few are not. Those outliers make it more difficult to find a specific grouping when quickly running an eye over the list to locate a variable of interest. Address this shortcoming by sorting the groupings alphabetically. NOTE: This change only sorts the top-level groupings (i.e. "core.*" comes after "completion.*"); it does not touch the ordering of variables within each group since variables within individual groups might intentionally be ordered in some other fashion (such as most-common-first or most-important-first). Reported-by: Bruno Haible <bruno@clisp.org> Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>